Optical multi-color monitoring of OJ 287 from 2006 to 2012

We present our optical multi-color monitoring of the BL Lac object OJ 287 from January 2006to December 2012 in the V, R and I bands. A relatively active state in OJ 287 has been found over allmonitored epochs, among which the variations of average magnitude in V/R/I bands were measuredwith △V = 1.956 mag, △R =2.067 mag and A1 =2.115 mag, respectively. No reliable intraday variabil-ity is detected, but possible variability is detected on 16 nights. Their relative variation amplitudes fallinto the range between 1% and 8%, with the majority between 2% and 4%. No time lags have been de-tected, but strong correlations exist among light curves in the three wavebands. The bluer-when-brightertrend is dominant over intraday timescales, which supports the shock-in-jet model. When combiningwith additional V/R band data obtained from SMARTS and the Steward Observatory, we also find abluer-when-brighter trend over a long-term timescale. Some possible periods of 513, 176, 36, 30, 26,17 and 14 d are found in all time-series data sets from 2006 to 2017. Possible explanations about theseperiods are given.