A GaAs defferential oscillator IC with on-chip LC resonator has been developed for suppressing the relative intensity noise (RIN) of a laser diode. The relationship between the Q-factor and minimum supply voltage for oscillation is fully described. In view of reducing the present LC resonator, we made use of BST (Barium Strontium Titanate) capacitor to make the resonator without increasing the chip area. The oscillation frequency is stable since it's determined by the geometry of the resonator. The experimentally fabricated oscillator IC achieved the output power of 12 dBm at the frequency of 600 MHz with voltage/current conditions of 2 V/20 mA. The present IC keeps quite stable RIN value less than -138 dB/Hz under the light-feedback condition up to 10%.