Polyurethane resin curable at low temperature and in a short time is prepared by contacting a compound having 1-oxa-3,5-diazine-2,4,6-trione ring as well as isocyanate groups, e.g. a reaction product of an aliphatic, alicyclic or aralkyl polyisocyanate, e.g. hexamethylene diisocyanate and carbon dioxide and a compound having active hydrogen, e.g. polyester polyol, with tert-amine, e.g. trimethylamine present in gaseous phase, at a temperature of 0-100°C. The ratio of the sum of the numbers of the trione ring and the isocyanate group to the number of the active hydrogen in the compound having the same is 0.1 to 10.