A process is described for preparing guanidine by separating guanidine from a process stream obtained in a urea synthesis process, wherein ammonia and carbon dioxide are reacted at a high temperature and a high pressure. The process stream includes a urea synthesis effluent from a urea synthesis zone, a solution obtained by separating unreacted materials from the effluent, a concentrate of the solution or a mother liquor derived from a urea crystallization zone.