In order to investigate an effect of non-protein nitrogen (NPN) on the biological utilization of protein, hatched single comb White Leghorn male chicks were fed for the first 8 days on a commercial chick mash, followed by 6 days on protein-free diet and the subsequent 6 days on the protein-free diets and protein diets containing 10.59% of crude protein supplemented with 0, 0.5, I.0 and I.5% of urea, respectively. When chicks fed protein-free diets, feed intake was lowered gradually as urea contents of diets were increased, while body weight loss was not altered. Protein efficiency ratio (PER) and net protein ratio (NPR) were increased in the order of the increasing level of urea in diet. Nitrogen balance were in negative balance while the values were increased with the increasing urea vontents in diets. Digestibility of nitrogen were lowered along with the increasing contents of urea, but biological value (BV) and net protein utilization (NPU) was found a trend to be high in birds fed 1.5% of urea. When birds fed protein diets, body weight gain and feed intake were not different among birds fed the graded levels of urea although feed conversions were trended to be high along with the increasing urea contents. The PER and NPR were lowered in the order of the increasing level of urea. Nitrogen balance and urinary nitrogen excretion was highered as urea levels of diets increased while fecal nitrogen excretion were not altered. BV and NPU of protein were found a tendency to be high in birds fed protein diets added with 0.5% of urea.