Objective: To further explore the mechanism of congenital pyrimidine 5′-nucleotidase Ⅰ deficiency.Methods: The samples were collected from the family members of a patient with P5′N-Ⅰ deficiency.The enzyme activities were measured by UMP method and the enzyme proteins were quantified by ELISA while the morphology of peripheral blood cells was observed.Results: The enzyme contents reduced as their enzyme activities decreased in the family especially in four members.There was a significant positive correlation(r=0.955)between the activity and the content of P 5′N-Ⅰ.The count of the stippling cell was varied in the family.Conclusion: One of the reasons for congenital P5′N-Ⅰ deficiency might be the deficiency in the enzyme content.The morphology of peripheral blood erythrocyte may be an assistant diagnotic index.The P5′N-Ⅰ activities and contents were measured simultaneously may be a effective method in clinic diagnosis.