We tested the effects of program trading to the stock market and the role of sidecar which is introduced to mitigate the inverse effect of program trading, using the daily and intraday order and trade data of the stocks listed in the KSE in the period of April to June, and September to October in 1999. In Korean stock market, the proportion of aggressive orders of program trading is very large despite the change of market conditions, but the trades induced by these orders do not seem to have information. In case of arbitrage trading, it does not have more information but have more liquidity consumption than the block trades. In case of non-arbitrage trading, it seems to have more information than the block trades in some cases, but it is hard to conclude. Volatility after program trading increases but disappears within 10 or 15 minutes, showing the market disturbance due to program trading is temporary. Liquidity imbalance due to program trading is temporary and does not have long-term effect.We can not conclude that sidecar stabilize the market and play the role that mitigates the information asymmetry. But it is shown that program trading is decreasing and the price discovery is delayed due to the exercise of sidecar.