The properties of electron states in the presence of microwave irradiation play a key role in understanding the oscillations of longitudinal resistance and the zero-resistance states in a high-mobility two-dimensional electron gas (2DEG) in low magnetic field. The properties of electron states in a high-mobility low-density GaAs/Al0.35Ga0.65As 2DEG in the presence of Ka-band microwave irradiation were studied by reflectance-based optically detected cyclotron resonance (RODCR). The influence of the direction of the microwave alternating electronic field, wavelength of the laser, and temperature on RODCR results was discussed. The results show that RODCR measurements provide a convenient and powerful method for studying electron states in 2DEG.