The purpose of this research is to examine and analyze the security return variability and trading volume activity before and after the stock split on companies listed at the Indonesia Stock Exchange in 2008, 2009, 2010, 2011 and 2012. This research using event study methodology. This type of research is the verificative research by using purposive sampling method. The sample of this research are all listed companies at the Indonesia Stock Exchange which have stock-split in the year 2008-2012. After being selected, there are 33 companies selected as sample of this research for the years 2008-2012. The data used in this research is secondary data which obtained from the audited financial statements for the fiscal year ended December 31, 2008, 2009, 2010, 2011 and 2012, also stock price, composite stock price index and the daily trading volume during the observation period which published by the stock market reference center at Indonesia Stock Exchange. The results of this research show that the security return variability there is no difference before and after the stock split, but trading volume activity there is difference before and after the stock split. Keywords: Stock split, security return variability and trading volume activity.