Thermal characteristics of single vertical-cavity surface-emitting laser (VCSEL) devices with different oxide apertures are studied experimentally in order to get better beam quality and higher output power by choosing appropriate size of the oxide aperture. Single devices with oxide apertures of 415, 386, and 316 μm are made by controlling the oxidation time. Diameters of the mesa and the P type contact are 450 μm and 400 μm, respectively. Thermal resistances of three kinds of devices are measured experimentally based on their different output characteristics under continuous-wave (CW) operation at room temperature. It is found that the smaller the oxide aperture is, the larger the thermal resistance of the device becomes. Temperature rise caused by self heating of injection current is obtained by comparing the relationship of current, wavelength and temperature. At a current of 1 A, the temperature of devices are 32.4 ℃, 35.2 ℃ and 76.4 ℃ corresponding to the oxide aperture of 415, 386, and 316 μm, respectively.