In Sri Lanka, there is not much evidence linked to stock split announcements and stock prices behaviour available to investors. This study, scrutinizes the stock price response to stock split declaration and a test of market efficiency in Colombo Stock exchange (CSE) by using a sample of 64 events (52 companies) from 14 different sectors of the emerging market during the period 2009 to 2012. Standard event study methodology is employed to find the results. The empirical results show that average abnormal return (1.46%) is statistically significant at 5% level on the stock split announcement day. This study finds that stock splits have a significant signal and information content in the Colombo Stock Exchange (CSE). On average, market positively reacts significantly to the announcement. Further, the large negative cumulative average abnormal return (-6%) is observed during the period of (0, 10). This results support the semi- strong form efficient market hypothesis for the sample companies within the study period since stock prices adjust so fast to public information that investor can not earn an abnormal return by trading in the stocks following the stock split announcement day.