Previous studies show investors’ overreaction towards firms past performance. In fact, investors overvalue past winners and undervalue past losers. However, when their expectations do not come true, stock prices return to their fair values. This study investigates investors’ overreaction in Tehran Stock Exchange in three steps. Using a five-year period data collected from Tehran Stock Exchange (TSE), we evaluate portfolio performance and analyze them based on a sample of 70 firms selected from this exchange and, using Pearson correlation as well as regression analysis, examine the effects of past performance on price appreciation. The preliminary results indicate that TSE investors normally overreact to sale, quality of sale, operating profit, quality of profit, cash flow and stock return, significantly.