Objective To explore the role of extracellular signal-regulated kinase (ERK) signaling pathway on cyclic strain induced proliferation of human periodontal ligament cell (hPDLCs). Methods The cultured hPDLCs in vitro were subjected to cyclic strain by FX-4000T system with 10 % or 20 %-elongation magnitude, 6 or 24 hours-duration respectively, at the same frequency of 0.1 Hz. The expressions of p-ERK1/2 and PCNA in hPDLCs were analyzed by Western blotting. To investigate the effect of p-ERK on cell proliferation, hPDLCs were incubated with PD98059, a specific ERK kinase inhibitor, and then the expression of PCNA was also analyzed by Western blot after hPDLCs were exposure to 10 %-cyclic strain at 0.1Hz-frequency for 6 hours. Result Cyclic strain increased the expression of PCNA and p-ERK1/2 in hPDLCs obviously, in which the magnitude and duration of cyclic strain had no significant difference. PD98059 could repress not only the activation of p-ERK1/2 but the expression of PCNA induced by cyclic strain in hPDLCs. Conclusion Cyclic strain promotes the proliferation of hPDLCs through ERK signaling pathway.