A process for treating water vapor generated in concentrating an aqueous urea solution wherein a urea synthesis effluent containing urea, unreacted ammonium carbamate and water from a urea synthesis zone is subjected to a plurality of decomposition stages, the pressures of which stages are stepwise reduced to decompose and separate substantially all of the unreacted ammonium carbamate from the aqueous urea solution. The aqueous urea solution which still contains small amounts of ammonia and carbon dioxide is concentrated to obtain crystalline urea or molten urea substantially free of water. The water vapor generated in concentrating said aqueous urea solution which contains small amounts of ammonia and carbon dioxide is cooled for condensation thereby forming a dilute aqueous ammonium carbamate solution which is subjected to rectification under a gauge pressure below 25 kg/cm.sup.2 to distil off a gaseous mixture of water vapor, ammonia and carbon dioxide, introducing the gaseous mixture into the unreacted ammonium carbamate decomposition stage using rectification under substantially the same pressure as the pressure of the rectification for the dilute aqueous ammonium carbamate solution to remove the water vapor from the gaseous mixture, and recovering the resultant ammonia and carbon dioxide together with a gaseous mixture of ammonia and carbon dioxide which is generated in the unreacted ammonium carbamate decomposition stage.