Spawn is the vegetative growth of the mycelium and serves as the inoculum or seed for cultivating mushrooms.Various cereal grains, such as wheat, millet, barley, sorghum, brown rice, rye, and oat were assessed to compare theircharacteristics and suitability for spawn production of button mushroom (Agaricus bisporus). The mycelial growth rates, density,and the number of completely colonized grains were measured from the twentieth day of inoculation. Wheat grains showedfastest mycelial growth with 8.4 cm followed by rye, oat, barley with 8.2, 7.5 and 7.3 cm, respectively. In the mycelial density,foxtail millet, barley, and sorghum were best compared with that of wheat grains. Especially, the number of grains which werecompletely colonized by mycelia were greatest in foxtail millet with 5,123 grains followed by proso millet, and wheat with 3,052and 914, respectively. Based on the results obtained, barley, foxtail millet, and sorghum grains would be appropriate substitutingfor wheat grain in spawn production of button mushroom.