Abstract: The RF Power Amplifier is an important component, whose stability can greatly affect the reliability of the communication system. Based on the self-designed RF-LDMOS device, a RF power amplifier was designed by ADS and its stability was studied. The R-C parallel circuit was added in the gate to improve the stability of the power amplifier in the 100MHz-200MHz band. Experience shows that the R-C parallel circuit performed as resistance when the frequency was under the transition frequency and performed as capacitance to the contrary. As a result, the R-C parallel circuit can effectively reduce the gain in low frequency with having no influence in the working frequency. The improved power amplifier has a stable gain of more than 17.4 dB in the 100 MHz-200 MHz band, and the output power of over 37.4 dBm.