The wavelength tuning of GaAs laser diode is performed by means of externl optical feedback from a grating mirror. The laser diode is anti-reflection coated, and the measured reflectivity is 0.012. The measured effective reflectivity of the external grating mirror is 0.06. As a result, tuning range of about 23nm is obtained and wavelength is tuned as dicrete longitudinal mode of the solitary laser diode. To interpret the experimental result, gain spectrum is measured by Hakki-Paoli method at various injection current levels. Based on the active Fabry-Perot modelling, the entire cavity is analyzed. The threshold gain decrease by the external grating feedback is about $37cm^{-1}$. The solitary laser diode may be lasing by external grating feedback within the band which has more than $-37cm^{-1}$ gain constant in the net gain spectrum curve. The caculated band is in good agreement with the experimental tuning range, 23 nm.
