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Introduction: In recent years, low-power lasers have been widely used in medicine. With the introduction of affordable light emitted diode (LED), clinical application of LED light has become more and more popular. However, some researchers believe that due to lack of coherence of the LED light, it can be different in terms of biological effects, in comparison with laser. In this study, the biological effects of low-level laser (LLL) to those of LED light are compared and discussed. Methods: Human skin fibroblast cell line Hu02 was irradiated with LLL and LED light with a wavelength of 660 nm, power output of 35 mW and in continuous mode and the control group was not irradiated. The biological effects were compared through analysis of cell proliferation, production of reactive oxygen species (ROS) within the cell and rate of cell division. Results: Our findings showed that production of ROS within the cell was linearly increased both in the LED and laser light irradiated cells. However, laser light is more incremental in comparison to LED light. The MTT results showed that laser light at low energy density (less than 5 J/cm 2 ) increased the rate of cell proliferation after 24 hours. Although, the rate of cell division was increased in energy density of 1 J/cm 2 compared to the control group, this increase was not statistically significant. Discussion: The findings indicated that the coherence properties of laser light provided more energy for the cells, and in a constant energy density, laser light created more oxidative stresses in comparison with LED light.