From March 3rd 2003, a new tick size policy of closed-fund has been taken into effect at Shanghai and Shenzhen Stock Exchanges, which changed from ＂1 fen＂ to ＂0.1 fen＂. The new policy is aimed to improve the closed-fund market liquidity and ＂splint arbitrage mode＂. By comparing the difference of three market quality measurement indicators including bid-ask spread, depth and volume before and after the tick size changed, we found that to some extent the ＂0.1 fen＂ tick size improved the closed-fund market quality such as liquidity and trading activity