This paper presents an analog predistortion circuit for RF optical fiber links. The circuit consists of two source-coupled differential transconductance amplifiers which could provide linear and nonlinear transfer charac-teristics by adjusting the bias voltage and the transistor sizes. The circuit was designed and realized in a standard 0.18-μm CMOS technology of SMIC. The chip occupies 0.48 × 0.24 mm~2. The DC supply is 3.3 V. Using this circuit, the third-order intermodulation distortion (IMD) suppression of a directly modulated RF optical fiber link can be improved by 9-16 dBc at relatively low cost.