We have tuned the lasing wavelength of a quantum dot laser diode (QDLD) by a thermal treatment. The InGaAs QDLD structure for 980 nm wavelength applications was grown by molecular beam epitaxy using the Stranski-Krastanov growth mode. The room temperature photoluminescence (PL) of a QDLD showed the ground state (GS) and excited state (ES) at the wavelengths at 993 and 946 nm, respectively. The 100 μ,m-wide and 4 mm-long broad area QDLD showed the lasing wavelength of 963 nm attributed to the ES of QDs with higher gain. After the thermal treatment at 800°C for 3 minutes with 300 nm-thick SiO 2 capping layers, the PL intensity of the GS increased, which caused the enhanced GS gain. The enhanced GS gain is thought to the attribution to the decreased carrier trapping due to the defects quenching. As a result, we could control the lasing wavelength of the QDLD from a wavelength of 963 nm to a wavelength of 980 nm. Moreover, the performances of these QDLDs have been discussed. This post-growth technique can be used to enhance the performances of the optoelectronic devices based on quantum dot.