The goal of this research work is to characterize and to suggest a method to improve the ten-layer InAs/InGaAs quantum dot (QD) laser performance by using post-growth thermal annealing. The approach of post-growth annealing is initially performed on the passive dots-in-a-well structure in this thesis. Interdiffusion of In and Ga atoms caused by thermal annealing has been proven from the photoluminescence measurements. The results show that p-doped QD structures are more resistant to intermixing. The post-growth thermal annealing technique is further applied to the active QD laser structure. Significant improvements in the performance of p-doped ten-layer InAs/InGaAs QD laser are demonstrated after annealing at 600 oC for 15 s. Defect reduction is thought to be the most likely mechanism contributing to the improved performance. The competition between the ground state (GS) and excited state (ES) lasing is investigated from the as-grown and thermally annealed QD lasers. Our results show that by adjusting the rapid thermal annealing temperature, the competition between the GS and ES lasing can be modified.