For a tunable diode laser system, Vortex from New focus, used in the present study, the linewidth is less than 300kHz for a sampling time of 50msec and the long term drift is typically of the order of 10GHz per a day. To stabilize the laser system, a servo controlled system has been employed. Rb atom and the nontunable cavity have been used as a reference and the laser diode has been used as a transducer. With a Rb vapor cell and a commercial lock circuit, we lock the laser frequency to one of the crossover signals of Rb atom. The frequency fluctuation is $Δν_rms = 1.18$MHz with a sampling time of 1sec and the lowest value of root Allan variance is $σ_y(t) = 50kHz$ for the integration time of t = 0.2sec. Cause the S/N ratio is only about 10 and the lock circuit was not optimized for the stabilization setup so that the high frequency noise could not be removed. Using the homemade lock circuit and the reference cavity, laser frequency was side locked to the cavity transmission signal. The frequency fluctuation is $Δν_rms = 52.5kHz$ for 20msec and 82.1kHz for 500msec, which is five to seven times narrower than that of free running laser. The calculation of Allan variance shows that the linewidth is narrowest (10kHz ) at short sampling time of t = 50msec and increases with longer sampling times. This is caused by the slow frequency drift of laser.