Some properties of xylanase produced by Pleurotus ostreatus during its gorwth in a rice straw medium were investigated. The results are summarized as follows : 1) The optimum pH of xylanase was 5.0 and the stable pH ranged from 4.5 to 6.0. 2) The optimum temperature for the xylanse was around $50^{circ}C$ and the xylanse activity was completely lost in 10 minutes at $70^{circ}C$ 3) The activity of xylanse was inhibited by managanous ion, but was increased by other metalic ions. Especially K, Mg and Ca ions considerably increased the activity.