  PKS 2155-304 is a well studied BL Lac object in the southern sky. The
historical optical data during different period have been collected and
compiled. Light curves with a time span of 35 years have been constructed. The
R-band light curve has been analyzed by means of three methods: epoch folding
method, Jurkevich method and discrete correlation function (DCF) method. It is
derived that there is an evident periodic component of 317 days (i.e. 0.87 yr)
superposed on a long-term trend with large-amplitude variation in the light
curve. The variability of this source is accompanied by a slight color
variation, and the brightness and color index are correlated with each other.
On the long time-scale, PKS 2155-304 exhibits a tendency of
bluer-when-brighter, which means the spectrum becomes flatter when the source
brightens.
