Laser frequency stabilization is important in laser science and various researches requiring laser beams with high frequency precision. However, the changes in physical parameters of the lasers themselves and in the external environments often cause instability in the output frequency of the laser. This thesis studied the frequency locking system for a continuous-wave diode laser. Using the Pound-Drever-Hall technique, the system can significantly reduce the linewidth of an input laser with an un-stabilized linewidth. It uses a high-finesse Fabry-Perot cavity, which is mechanically and thermally isolated, as a frequency reference to measure the time-varying frequency of the input laser. An electronic feedback loop works to correct the frequency error and maintain constant optical power.