Oyster Mushroom (Pleurotuse ostreatus) is one of the industrial, pharmaceutical and commercial edible basidiomycetes. In this study, we examined colchicine treatment for the production of autopolyploid P. ostreatus. For production of mycelium, the spores were cultured on MEA culture medium. For colchicine treatment, the mycelia were then cultured in Czapek liquid culture medium containing glucose (0.1% w v-1) and peptone (0.5% w v-1) adding 0, 0.05, 0.1 and 0.3% of colchicine solutions to the medium. The mycelia were stained with Safranin-O and their nuclei diameters were measured, using IX170 Olympus invert light microscope interfaced with a DP50 digital camera. The nuclei were increased in diameter at different degrees in different concentration of colchicine solutions. Such increment response indicates the increase of nuclei DNA content, verifying the autopolyploid formation of P. ostreatus. From the produced mycelia spawn were made and in order to evaluate some important morphological characteristics, they were used for inoculating wheat straw. After the start of fruiting stage, yield, diameter of basidiocarp and length of stem were measured. The yield was increased up to %82 compared to the control, the stem and the diameter of basidiocarp also enlarged in the colchicines-treated P. ostreatus.