Abstract: Differential pressure method was proposed for determination of hydrolysis rate constant of isocyanate. Carbon dioxide and ammonia are generated by the reaction of tetraisocyanato-silane and water. Ammonia was absorbed by 2 mol·L-1 H2SO4 solution and the raise of pressure due to carbon dioxide was measured by U-manometer. It was found that in testing for the hydrolysis of tetraisocyanato-silane with its concentrations of 0.006 mol·L-1 and 0.010 mol·L-1, magnitude of increase of pressure kept linear relationship with the time of reaction which attained to an equilibrium point at 30 min and 38 min respectively, denoting the completion of hydrolysis. Values of correlation coefficients found were 0.992 8 and 0.984 9 respectively. On the base of the experimental data, hydrolysis rate constant of tetraisocyanato-silane was found to be 0.102 s-1 with its half-life of 6.8 min.