The aim of this study was to investigate the effects of UV-B irradiation on the physicochemical characteristics of oyster mushrooms (Pleurotus ostreatus). Vitamin $D_2$ concentration, weight loss rate, color value, total plate counts and consumer acceptability of irradiated oyster mushrooms were measured. UV-B irradiation at doses of $0;kj/m^2$, $20;kj/m^2$ and $40;kj/m^2$, significantly increased the vitamin $D_2$ concentrations from $0;{mu}g/g$ dry weight (control) to $85.87;{mu}g/g$ dw and $116.28;{mu}g/g$ dw, respectively, at 5% level. Rate of weight loss was also significantly increased from 0% (control) to 2.21% and 4.31% at $20;kj/m^2$ and $40;kj/m^2$ UV-B irradiation, respectively, at 5% level. Although there was no significant difference between the UV-B irradiated groups, total plate counts were significantly decreased from $1.0{ imes}10^5$ (control) to $9.4{ imes}10^3$ and $1.9{ imes}10^3$at $20;kj/m^2$ and $40;kj/m^2$ of UV-B irradiation, respectively, at 5% level. There was no significant difference in L values and consumer acceptability between the groups. Therefore, UV-B irradiated oyster mushrooms could be used as health promoting ingredients for many foods.