Abstract： Objective To approach the mechanisms of effects of high glucose on the differentiation of periodontal ligament cells (PDLC) by investigating the changes of Scleraxis mRNA expression in high glucose condition in vitro. Methods Human PDLC were cultured in high glucose medium (4500 mg/L glucese) and normal glucose medium (1000 mg/L glucose), respectively. High glucose was uesd to inhibit the ostengenic differentiation of PDLC. PDLC cultured in normal glucose medium served as controL. Alkaline phosphatase(ALP) activity, the early parameter of osteogenetic differentiation of cells and the expression of Scleraxis mRNA were detected in each group. ALP activity was measured colourmetrically by using nitrophenyl phosphate as a substrate and the expression of Scleraxis mRNA was analyzed by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). Results ALP activity of PDLC was lower in high glucose medium than in normal glucose medium, and the values were 0.113 ± 0.068 and 0.218 ± 0.012, respectively. However, the level of Scleraxis mRNA was quite higher in high glucose midium compared with in normal glucose medium, and the values were 0.973 ± 0.055 and 0.611 ± 0.205,respectively. The values of ALP activity and the expression of Scleraxis mRNA were significantly different between the two groups. Conclusions High glucose inhibited osteogenetic differentiation of PDLC and up-regulated Scleraxis expression. The adverse changes of Scleraxis expression and osteogenic differentiation of PDLC suggest that Scleraxis may regulate the esteogenic differentiation of PDLC negatively and the inhibition of high glucose on osteogenetic differentiation of PDLC may be regulated by Scleraxis in transcription level.