Triethylgermyldiphenylphosphine was reacted with phenylisocyanate at various temperatures for three days in sealed ampoules. At $0^{circ}C$, only N, N-diphenyluretidine-2,4-dione, a cyclic dimer of phenylisocyanate (35%), was formed. But at $20^{circ}C$, phenylisocyanutrate, a cyclic trimer of phenylisocyanate (30%), was formed along with the dimer. At $50^{circ}C$, diphenylcarbodiimide (55%) was given together with the compounds described above. At the higher reaction temperatures than $100^{circ}C$, instead of the dimer and trimer of phenylisocyanate, 1,3,5-triphenyl-2,4,6-tris(phenylimino) hexahydro-1,3,5-triazine, a cyclic trimer of diphenylcarbodiimide (30%) and diphenylcarbodiimide (70%) were mainly produced. Triethylgermyldiphenylphosphine appears to act as a catalyst for the formation of the above cyclic compounds.