Objective To study the expression of caspases in stretch-induced apoptosis in human periodontal ligament cells (HPDLCs). Methods HPDLCs in vitro were subjected to mechanical stretch with 20% strain for 6 h or 24 h. The apoptotic rates were analyzed by flow cytometry. The protein expression of caspase-3, -5, -7, -8 and -9 was detected by Western blot, and the activity of caspase-3, -5, -8 and -9 was measured using colorimetric assay. Results Mechanical stretches with 20% strain for 6 and 24 h could induce apoptosis in HPDLCs. Compared with non-stretching control group, the protein expression level and activity of caspase-3, as well as the protein expression level of caspase-7 were up-regulated by 24 h-stretch. The protein expression level and activity of caspase-5, -8, -9 were up-regulated after stretches for 6 h and 24 h. Conclusions Mechanical stretch with 20% strain can induce apoptosis in HPDLCs in vitro, with the activation of caspase-3, -5, -7, -8 and -9.