This study investigated the effect of UV-B irradiation on the quality of Pleurotus ostreatus. The changes of vitamin $D_2$ contents, color value and flavor pattern in mushrooms were analyzed by high-performance liquid chromatography (HPLC), chromameter and gas chromatography - surface acoustic wave (GC-SAW) electronic nose. By exposure to UV-B irradiation (0 kj/m$^2$, 10 kj/m$^2$, 20 kj/m$^2$), vitamin $D_2$ content increased from 0 (control) to 48.50 g/g (DM: dry matter, 10 kj/m$^2$) and 61.58 g/g (DM, 20 kj/m$^2$). Although there was no significant difference in L, a, b values among the three groups, flavor changes were detected by GC-SAW electronic nose. The number of peaks increased from 10 in the control group (0 kj/m$^2$), to 14 and 15 for the 10 kj/m$^2$ and 20 kj/m$^2$ groups, respectively. Nevertheless, the changes of flavor pattern were not detrimental to the mushroom quality. These results suggested that UV-B irradiation is an effective method to increase the vitamin $D_2$ content without degrading the quality.