Final volatiles yield of biomass devolatization at flash heating rate was determined via experiments. Samples of wheat straw and corn stalk powder were dispersed in the feeder and were carried by argon into the laminar entrained flow reactor. Flash devolatization of tiny biomass particles was accomplished in the moderate temperature reactor. Residue was collected and the ash content of it was detected in a thermo-grametric analyzer. Final volatiles yield was calculated using ash trace method. Altogether, 96 experiments (including repeat ones) of different reaction temperature (500, 550, 600, 650 °C), different collecting distance (550, 600, 650, 700 mm away from inlet). Results showed that final volatiles yields of wheat straw and corn stalk are 79%, 85 % separately. High reproducibility of the experiments showed it is a reliable method to determine final volatiles yield of biomass devolatization at flash heating rate. Kinetic parameters for wheat straw and corn stalk were modified as A=1009/s, E=3782kJ/mol and A=596/s, E=3716kJ/mol, respectively.