In this study, we investigated the optical properties of LED light in continuous-wave and pulsed modes to elucidate the effects of LED light in inhibiting the formation of cariogenic Streptococcus mutans biofilms and promoting the activation of mouse fibroblast (L929) intraoral normal cells. LED modules emitting wavelengths from 390 to 850nm were used to irradiate S. mutans or mouse fibroblasts in cell plates, and the cell viability was observed. The optical properties were calculated by analyzing the horizontal irradiance on the bottom surface of the cell plate. When irradiated by the 630nm LED module at a radiant exposure of 2.3J/㎠, the mouse fibroblast viability increased by 43% and the S. mutans biofilm-formation inhibition rate was 50%. In the test of the 405nm LED, we confirmed that even if the radiant exposure is reduced, the S. mutans-formation inhibition rate can be improved if the irradiance is high. Pulsed light outperformed the continuous wave in enhancing mouse fibroblast viability, and it was more effective when the pulse width was widened with the pulse period unchanged.