Steam gasification or pyrolysis of biomass was conducted at heating rates of 1, 10, and 100 K S-1 using a thermobalance reactor. Three kinds of biomass samples (cellulose, lignin, and bagasse) were used. Time profiles of weight change of biomass samples during steam gasification and pyrolysis were measured at 973 K. Char obtained in steam gasification of lignin was analyzed with a scanning electron microscope and a CHNS elemental analyzer. The effect of the heating rate on final conversion and reaction rate of char was investigated. It was found that a higher heating rate substantially increased the reaction rate of lignin char in steam gasification because porous char was produced during devolatilization due to rapid evolution of volatiles. A higher heating rate also increased final conversion of biomass. The heating rate showed no pronounced influence on elemental composition of char.