Abstract： Objective To investigate the effects of cyclic tensile strain on β- actin mRNA expression in articular chondrocytes.Methods The normal and articular chondrocytes with osteoarthritis were obtained and exposed to 3% , 6% and 15% cyclic tensile strains, respectively.The real-time quantitative PCR was used to determine the altered expressions of the P-actin mRNA under different cyclic tensile strains.Results Both 6% and 15% cyclic tensile strains significantly up-regulated the β-actin mRNA expressions (1.34±0.05, 1.36±0.04, P＜0.05) in normal articular chondrocytes, whereas only 15% cyclic tensile strain was shown to be associated with up-regulated β-actin mRNA expressions in articular chondrocytes with osteoarthritis.Conclusions The β-actin mRNA expressions of articular chondrocytes may change along with different level of cyclic tensile strains.Reaction of articular chondrocytes to cyclic tensile strains may vary under different internal or external circumstances.Therefore, the β-actin appears ineligible as a housekeeping gene in studies on biomechanical of chondrocytes.