Four different rice straw substrate volumes (0.5, 1, 2 and 5 kgs) were assessed for cultivation of oyster mushroom (Pleurotus ostreatus). The mushroom grown in 2 kgs substrate volume exhibited the highest biological efficiency and produced the highest fruiting bodies yield. This treatment gave the greatest average weight for the whole fruiting body and for the fruiting body cap. However, mushroom grown in 1 kg substrate volume was earlier than 2 kgs volume to colonize. Both treatments were statistically alike concerning days lapsed to harvest the fruiting bodies, the diameter of the fruiting bodies and the weight and the length of the stem. Further, the mushroom in these treatments degraded the least amount of substrate as shown by both the colonized substrate and spent weight while producing high fruiting bodies yield. Thus a more efficient bioconversion is suggested for the mushroom grown in 1 or 2 kgs substrate. The overall data, however, propose the production of the oyster mushroom (Pleurotus ostreatus) in substrate media of 2 kgs volume.