Laser produces a monochormatic collimated and coherent radiation. In dentistry, diode lasers have been used predominantly for application which are broadly termed "Low level laser therapy (LLLT) or biostimulation (L.J. Walch 1997)". Periodontal ligament fibroblast (PDLF) have a key function in periodontal regeneration. Stimulatory effects on the proliferation of these cells could therefore be beneficial for the reestablishment of connective tissue attachment. The aim of this in vitro study was to evaluate the potential stimulatory effect of low level laser irradiation on the proliferation of PDLF.