Abstract: To learn the variation in the gene for UGT1A1 enzyme, the genetic mechanism in a Chinese Han nationality family suffered from Gilbert's syndrome was studied. At first, genomic DNA from peripheral blood of the sufferer in this family was used for amplifying all of the five exons of the UGT1A1 gene by PCR, and then direct sequencing of the PCR product was applied to analyze gene mutation. The results showed that there existed a G-->A homozygous transition at nucleotide 211 leading the substitution of arginine for glycine at position 71 of corresponding protein product (G71R) and a T-->G homozygous transition at nucleotide 1456 leading the substitution of aspartic acid for tyrosine at position 486 of corresponding protein product (Y486D). No mutation was detected in promoter region and the splicing junction sites. The relevant mutation sites of the other family members were sequenced and identified to be heterozygous in the two above-mentioned mutation sites and in the TA repeat mutation in the promoter region. Furthermore, fresh blood samples were collected from all of the members to detect the serum bilirubin levels to determine the sufferer. The result was consistent with the mutation analysis. It could thus be inferred that this family was caused by mutation in the open reading frame of the gene UGT1A1.