Glioblastoma is an extremely aggressive brain tumor with barely minimal chances of treatment. The effect of low level light on cell cycle and proliferation of different cell lines have already been reported. In the present work, the effect of low level light therapy using light with wavelength of 530nm on the U87 cell line viability is being investigated. In this study, the cells were treated with 15 minutes irradiation from green LEDs with an average power of 1.12mW/cm². The cell viability during a 24 hours period was measured using colorimetric (MTT) assay at different time laps. The results indicate a time dependent decrease in the cells activities.