The Oyster mushroom (Pleurotus ostreatus Jacq.) is one of the mushrooms that is easily cultivated. Most Indonesian mushroom farmers are using sawdust as a base material for growth media of oyster mushroom. In an effort to prevent decrease in the availability of sawdust, alternative base materials are needed to reduce the cost. This research aims to determine the mycelium growth on every tested substrate and to know the effect of rice bran addition in oyster mushroom's mycelium growth rate. This research used sawdust, grass straw, and rice husk as the substrates. The effect of the presence of rice bran was done by adding rice bran to the substrate. This research used treatment composition as follows: M1 (sawdust 800 g and rice bran 204 g), M2 (sawdust 800 g), M3 (grass straw 400 g and rice bran 204 g), M4 (grass straw 400 g), M5 (rice husk 800 g and rice bran 204 g), and M6 (rice husk 800 g). The results showed that M2, M4, and M6 have mycelium 44.37 %, 28.54 % and 24.58 % respectively in 21 DAI. Our results indicate that we can use sawdust without adding rice bran as the base material to reduce the cost or substitute the sawdust with grass straw and without added rice bran to be an alternative base material for growth media.