| In this thesis, we have summarized the basic properties, major characteristics, the results of the new observations and theoretical studies about the active galactic nuclei (AGNs). Especially, we detailed review the special sub-class objects: blazers of active galactic nuclei, focus on observations, timescale variability, properties of multi-band spectral energy distributions (SED), and the theory model of blazers. The thesis also summarizes the some studies and the new results of research of active galactic nuclei during my PhD in observations, timescale variability, medium-timescale periodicity of the variability, central structure and supper-massive black hole (SMBH), and the some studies for evolutionary sequences and grand unification of the AGNs.Our main works and results are as follow: (1) studying on short timescale variability and central supper-massive black hole (SMBH). The observational results show that the short timescale variability is a notable property for blazar observation. According to the theory model of short timescale variability and central supper-massive black hole (SMBH), we observed the important results in estimatingthe mass of SMBHs by using 29 GeV- γ -Loud Blazar samples. Our result isconsistent with the results of Kormendy & Richston (1995, 1998) and Barth et al (2003). (2) medium-timescale periodicity of the variability in blazers. We find that variabilities on a medium time-scale of one or several months were observed in some blazars. We have presented the historical light curve in I-band of the BL Lac object S5 0716+714 and searching for its possible medium-timescale periodicity using the Jurevich and power spectrum method. Our results show that this BL Lac object probably has a medium-timescale periodic activities, which period is of around 14.0±0.1 days in the both different method analysis, and these results support theresults obtained by Impey et al(2000) and Qian et al(2003). (3) Research work for central structure and supper-massive black hole (SMBH), We use a large sample ofGeV- y -Loud Blazar, for which rapid optical variabilities and Y -ray fluxes are wellobserved, has been compiled from the literature. Estimates for the masses, the minimum Kerr parameters, and the size of the emission regions, of supper-massive black holes (SMBHs) for the blazars in the sample have been derived from theirminimum optical variation timescales and Y -rav fluxes. (4) The studies forevolutionary sequences and grand unification of the AGNs. The large sample includes 132 radio quasars (103 RLQs and 29 RQQs), 81 Seyfert galaxies, 21 BL Lac objects, and 18 FSRQs, 53 FRII and 26 FRI radio galaxies. We will study deeply this question quantitatively with four key parameters of galaxy: the mass of central SMBHs, bolometric luminosity, accretion rare and the angular momentum per unit mass of theblack hole. We can see that the quasars with low value of a/M < 0.75 evolve into aBL Lac objects and then BL Lac object may evolve into a FR radio galaxy (FRII or FRI). In addition, another evolutionary track of AGNs is from radio QSOs to Seyfert galaxies. This work explains successfully that existence of two evolutionary sequences for AGNs, supporting the evolutionary sequence proposed theories by Punsly (1996), developing the studies for evolutionary sequences and grand unification of the AGNs.There are six chapters in this thesis altogether. Chapter one is an overview of basic properties and major characteristics about the active galactic nuclei (AGNs). Chapter two presents observation and timescale variability of blazers. Chapter three introduces some theory model and spectral energy distributions of blazars. Chapter four presents medium-timescale periodicity of the variability in blazars. Chapter five introduces some researching work for central structure and supper-massive black hole (SMBH) of blazer. Chapter six introduces some researching work for evolutionary sequences and grand unification of the AGNs.
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