This trail was conducted to study the effects of dietary supplementation of melamine byproducts on residual concentration of melamine in rumen fluid, blood, urine, feces and milk of dairy cows. The further object was to reveal the metabolic mechanism of melamine in dairy cows by single factor experiment. The supplemental levels in the concentrate were 0.516%, 0.860% and 1.204%, respectively. The experiment was divided into 3 phases with 10 days in each phase (7 days for pre-feeding and 3 days for sampling). The same 5 Holstein dairy cows were fed the same experimental diet in each phase. The results showed that the residual concentration of melamine in milk, feces and urine were all increased with the growing supplementation level. The concentration of melamine in the raw milk, feces and urine from the phase with the highest supplemental level were 1.31, 3.30 and 107.75 mg/kg, respectively. It is concluded that renal excretion is the primary metabolic pathway of melamine while defecation and lactation are only auxiliary pathways. ［Chinese Journal of Animal Nutrition, 2010，22（6）:1703-1708］