Optical, spectral, and thermal characteristics of InGaN/GaN multiple quantum well green light-emitting diodes (LEDs) with a large chip size of 0.8 × 1 mm2, operating at λ ∼ 525 nm, on patterned sapphire substrate (PSS) were studied. The temperature-dependent optical and spectral properties of LEDs were measured and analyzed. The junction temperature (Tj) was also determined by the forward voltage method, in comparison with the simulation results which were theoretically calculated by a three-dimensional anisotropic heat dissipation model based on the finite element method. Under an injection current of 350 mA at 298 K, the optical output power and forward voltage were obtained to be 63.5 mW and 3.98 V, respectively. The characteristic temperature was found to be ∼438 K at an injection current of 350 mA. The Tj was increased with increasing the injection current. From the measured Tj values, the thermal resistance (Rth) value was experimentally obtained to be ∼6.31 K W−1, which was well consistent with the simulated result (Rth ∼ 6.24 K W−1).