An optical transmitter is disclosed, where the optical transmitter shows an emission wavelength kept stable in one of grid wavelengths of the WDM system during the boot of the transmitter. The optical transmitter includes an LD, a TEC to control a temperature of the LD, and a controller. Detecting the flag to enable the optical output, the controller increases the driving current of the LD concurrently with the decrease of the temperature of the TEC to compensate the self-heating of the LD due to the driving current.