Glass transition temperatures of water-blown rigid polyurethane foams at three levels of initial water content, 4.5-5.5%, and five levels of soy flour, 0-40%, were determined by dynamic-mechanical instrumentation at the temperature range of 50-280 degreesC and the frequencies from 0.1 to 20 Hz. The results showed that both the addition of soy flour in the rigid polyurethane foam system and increasing initial water content contributed to a higher glass transition temperature. Moreover, increasing the percentage of the soy flour in the rigid polyurethane foam system led to higher compressive strength and broader master curves for the imaginary part of the elastic modulus. The KWW function fitted the master curve better than the HN and CD functions.