Radiation from the blazar class of active galactic nuclei ( AGN ) exhibits fast time variability which is usually ascribed to instabilities in the emission region near the central supermassive black hole . The variability time scale is generally faster in higher energy region , and data recently provided by the Fermi Gamma-ray Space Telescope in the GeV energy band enable a detailed study of the temporal behavior of AGN . Due to its wide field-of-view in the scanning mode , most sky regions are observed for several hours per day and daily light curves of many AGN have been accumulated for more than 4 r . In this paper we investigate the time variability of 15 well-detected AGNs by studying the normalized power spectrum density of their light curves in the GeV energy band . One source , 3C 454.3 , shows a specific time scale of 6.8 \times 10 ^ { 5 } s , and this value suggests , assuming the internal shock model , a mass for the central black hole of ( 10 ^ { 8 } – 10 ^ { 10 } ) M _ { \odot } which is consistent with other estimates . It also indicates the typical time interval of ejected blobs is ( 7 – 70 ) times the light crossing time of the Schwarzschild radius .