High output power values of 15.7 mW at 20 °C and 2.7 mW at 110 °C were obtained from a blue GaN-based vertical-cavity surface-emitting laser (VCSEL) under continuous-wave operation as a result of introducing a long-cavity (10λ) structure. The threshold current and voltage at 20 °C were 4.5 mA and 5.1 V, respectively. Owing to the reduced thermal resistance provided by the long-cavity structure and the adjusted reflectivity of the front cavity mirror, this VCSEL also exhibited a high slope efficiency of 0.87 W/A, a differential quantum efficiency of 31%, and a wall-plug efficiency of 8.9%.