In making investments, especially stocks, the relevant information is crucial decisions for investors. However, in making investment decisions, the relevant information is not only one factors that influence an investor investment. There is another factors as emotion, everyday behavior, situations, conditions and other matters stick inside an investor that also can affecting investment decision-making, this is called the science of Behavioral Finance. The phenomenon of overreaction is a situation where stocks that have poor performance (loser) in the future will be become a stocks that have performed well and stocks are performing well (winner) in the future will be become a stocks that perform poorly. January effect is a market anomaly that occurred in January where there is a higher rate of return in January then other months. This study aimed to analyze the effect of the January effect to overreaction that occurs in the Indonesia Stock Exchange. The samples used are shares listed on the Indonesia Stock Exchange in category LQ 45 from the year 2007 to 2014. The method used is purposive sampling by taking samples in the first semester in basis year of samples. Overreaction tested in monthly span of two formations-observation. January effect test based on the stock return and abnormal return. Based on the results of research conducted that there is separatist overreaction occurred at Indonesia Stock Exchange during the observation period. But the January effect does not occur at Indonesia Stock Exchange. Based on the results that have been tested that the January effect does not affect to overreaction is seen from the January effect tested that does not occur in Indonesia Stock Exchange. 
Keywords: Overreaction, January effect, Index LQ 45