Improvement of phytase production from Saccharomyces cerevisiae CY strain was attempted by optimizing the culture medium. The optimum medium components for cell growth and enzyme production were found to be galactose, ammonium sulfate, sodium phytate, and magnesium sulfate. Galactose (5%) and ammonium sulfate (2%) were the most suitable carbon and nitrogen sources, respectively, for phytase production. The phytase activity was most active at pH 4.0 in the initial medium conditions. Phytate stimulated enzyme production at low concentration; however, phytase activities were decreased with increase of phytate concentration at above 0.002%. Maximum phytase activity was 135.09 mU/mg DCW after 72 h fermentation under optimal conditions. The harvested S. cerevisiae CY cells were used for the degradation of phytate in the soybean-curd whey. Over 60% of phytate in the soybean-curd whey were nearly linearly degraded with cells of 27.16 U/g-phytate dosage as phytase source after a hydrolysis time of 2 h.