Objective:To reevaluate the upper limit of normal (ULN) of serum alanine aminotransferase (ALT) by retrospectively analyzing the ALT levels in healthy people in Ningbo area.Methods:A total of 56 140 people who underwent health examination and detection of liver biochemical indexes in the Affiliated Hospital of Medical School of Ningbo University and Yinzhou Huamao Hospital of Ningbo from 2018 to 2020 were enrolled. After excluding relevant factors that may lead to liver injury, 11 411 people were included to compare the difference of serum ALT levels among different genders and age groups (20 to 29 years, 30 to 39 years, 40 to 49 years and 50 to 59 years) to determine the ALT ULN in different gender groups. Statistical methods were performed using two independent samples t test and analysis of variance. Results:The serum ALT of males was (19.20±7.90) U/L, which was higher than that of females ((13.75±6.17) U/L), with statistical significance ( t=41.16, P<0.001). The serum ALT ULN in males and in females were 35 U/L and 26 U/L, respectively. The serum ALT levels of 20 to 29, 30 to 39, 40 to 49 and 50 to 59 years old groups were (15.48±7.61) U/L, (16.21±7.40) U/L, (17.36±7.52) U/L and (18.77±7.57) U/L, respectively.The difference was statistically significant ( F=71.51, P<0.001). Serum ALT level in 50 to 59 years old group was higher than that in 20 to 29 years old group, and the difference was statistically significant ( t=13.11, P<0.01). In males, the ALT ULN of 20 to 29 years old was the lowest of 34.43 U/L, and highest of 35.29 U/L in 40 to 49 years old. In females, the ALT ULN in the 20 to 29 years old group was the lowest of 23.01 U/L, and the ALT ULN in the 50 to 59 years old group was the highest of 30.79 U/L. ALT ULN increased with age in females. The serum ALT of males was higher than that of females in all age groups ( t=29.55, 26.91, 13.43 and 4.62, respectively, all P<0.05). Conclusions:The serum ALT level is significantly correlated to gender and age. The serum ALT ULNs of healthy adult are 35 U/L in males and 26 U/L in females in Ningbo area.