Disclosed is a novel method of controlling the formation of biuret in urea production. This is accomplished by reducing or preventing the formation of biuret in a concentration section, particularly in one or more concentrators or evaporators. The method comprises controlling the residence time of a urea aqueous stream treated in such concentration section in a manner independently of the volume flow per time interval of said stream into said concentration section. The residence time can be controlled, e.g., by providing the concentration section with an adjustable volume or by adding a gas to the urea stream to be treated. A combination of such measures can also be applied.