Abstract: To investigate the methods to effectively and simply assess the CAG repeat size of HD gene which was necessary for gene diagnosis of Huntington disease, the sequence including polymorphic CAG repeat of HD gene was amplified by PCR with TaKaRa LA Taq DNA polymerase and GC buffer. PCR products were analyzed on polyacrylamide gel to distinguish normal alleles from HD alleles. The DNA fragments of affected alleles were recovered from polyacrylamide gel as templets for secondary PCR. The secondary PCR products were cloned into T vector for sequencing analysis to determine CAG repeat size. A total of 20 normal individuals and 3 members from a HD pedigree were included in this study. The results showed that the CAG repeat numbers in 20 normal individuals and 3 normal alleles from a HD pedigree varied in normal range, while in 3 HD alleles, the copy number of CAG repeat were 39, 40, 41 respectively. There was no overlap between the copy number of the normal and affected alleles. In conclusion, the TaKaRa LA Taq DNA polymerase with GC buffer can be used to effectively amplified CAG repeat of HD gene, which combined polyacrylamide gel electrophoresis and DNA sequencing analysis can diagnose HD accurately. In addition, these finding suggested that the dynamic mutation in HD gene be responsible for the genetic defect in Chinese HD patients.