The optical properties of thermally annealed InGaN/GaN multiple quantum wells were investigated by low-temperature photoluminescence measurements. It is found that the photoluminescence peak exhibits a redshift followed by a blueshift as the annealing time is increased. In contrast, the assigned photoluminescence peak from an In-rich dot-like structure shows a monotonic blueshift with more annealing time. Transmission electron microscopic observation confirms that the density of dot-like structures is reduced after thermal annealing, indicating that phase separation does not take place in these samples. Instead, in-plane and out-plane outdiffusion of dot-like structures is proposed to account for the spectral shift with more annealing time. Based on this diffusion model, a quantized state transition in the quantum well along with the composition inhomogeneity and piezoelectric field is considered to be the dominant luminescence mechanism.