Background and Aims:Red laser light of wavelength 630 nm is usually used for Photofrin ® -mediated photodynamic therapy (PDT). The 630-nm light employed in PDT corresponds to the region of the wavelength used in low-level laser therapy (LLLT) may influence on the photodynamic effect required for killing cancer cells. The aim of this in vitro study was to investigate the changes in cell viability and degree of cell proliferation after Photofrin ® -mediated PDT using 630-nm pulsed laser irradiation (10 Hz repetition rate and 7-9 ns pulse width), which was clinically found to induce no remarkable cell injury. Materials and Methods: A study has been conducted in which HeLa cells are incubated with Photofrin ® for 15 min (10 µg/ml). Irradiation was carried out at an average fluence rate of 50 mW/cm 2 with light doses of 1, 3, and 5 J/cm 2 . The cytotoxic effects on the cells are evaluated by the XTT (2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide) assay. Results:The results showed that the laser irradiated cells exhibited a greater clonogenic activity than normal and PDT treated cells for a short period after the laser irradiation. Conclusion: If the level of 630-nm pulsed laser irradiation employed in a PDT is comparatively lowered, it would have a biostimulatory effect like that of in LLLT.