Photobiomodulation is recognized as an effective method for adjunct therapy in periodontal treatments. our purpose in this study was to investigate the effects of different energy densities of 915nm diode laser on the viability and viability capacity of human gingival fibroblast cells. cell samples were examined in 5 groups, including 4 irradiation groups with low-level diode laser 915nm, 1,2,3,4 J/cm2, and a control group (no Laser irradiation). Cell viability and viability were measured 1, 3 and 5 days after irradiation by MTT and DAPI assay. Statistical differences between groups at any time were analyzed by one-way ANOVA and a post hoc Turkey’s test. The cell viability and viability capacity increased on the third day at an energy density of 3 J/cm2; (P-value = 0.007) and the fifth day at energy densities of 2,3 and 4 J/cm2 was recorded compared to the control group (P-value =0.000). Also, a significant decrease in the viability and viability of irradiated cells with an energy density of 1 J/cm2 was found. (P-value =0.033). according to our results, Photobiomodulation with 915nm diode laser has a positive stimulating effect on the viability and viability capacity of human gingival fibroblast cells.