Objective: The present study investigates whether the exposure to low-power diode laser induces denaturation in red blood cell (RBC) membrane protein composition, and determines the irradiation time for when denaturation of membrane protein process begins. Background: A low-energy laser has been used extensively in medical applications. Several studies indicated significant positive effects of laser therapy on biological systems. In contrast, other studies reported that laser induced unwanted changes in cell structure and biological systems. The present work studied the effect of irradiation time of low-power diode laser on the structure of membrane proteins of human RBCs. Materials and methods: The RBC suspension was divided into five equal aliquots. One aliquot served as control. The remaining four aliquots were exposed to low-power diode laser (wave length = 650 nm, power = 50 mW) for 10, 20, 30, and 40 min, respectively. After each given time, the percentage of denatured RBCs was calculated in each sample as described later. Results: Irradiation of RBCs by this laser for 20 min did not cause any change in membrane protein composition. While increasing the irradiation time to 30 min caused denaturation of membrane proteins, resulting in the formation of cross-bonding in a considerable number of RBCs, and the percentage of denatured cells increased in a dose-dependent manner to the irradiation. Conclusions: It can be concluded that the effect of low-power diode laser on RBC membrane protein structure depends on irradiation time.