Reactions of trimethylsilyldiphenylphosphine with phenylisocyanate and ethylisocyanate were studied at various temperatures for 3 days. Trimethylsilyldiphenylphosphine gave cyclic dimer, cyclic trimer and diphenylcarbodiimide from phenylisocyanate, but gave cyclic trimer, triethylisocyanurate, only from ethylisocyanate. In these reactions, it is suggested that trimethylsilyldiphenylphosphine was used to be an effective catalyst for cyclization of isocyanates.