Guanosine diphosphate mannose (GDPM), coenzyme of mannose polymerization, was chemically synthesized. GMP and cyclohexylamine were condensed to form N-cyclohexylphosphoramidate in the presence of DCC. The amidate (III) was reacted with benzyl hydrogen phosphate and after hydrogenation led to GDP (V). (III) and α-d-mannos-1-yl phosphate were condensed in pyridine and, after isolation and purification by ion-exchange chromatography, 83.5% pure GDPM was obtained in 32% yield. The synthetic GDPM was quite identical with the natural one in all respects.