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Based on TSMC 0.18ȝm RF CMOS process, a 4thorder Butterworth low-pass filter (LPF) used in a multi-mode and multi-standard wireless receiver (for WCDMA and GPS/Galileo) is presented in this paper. The LPF adopts activeg m -RC structure, and can satisfy both WCDMA and GPS/Galileo bandwidth requirements, which maximizes hardware reuse. In order to improve the accuracy of the bandwidth, a RC time constant auto-tuning circuit is applied. The layout of the LPF with auto-tuning circuit occupies a 900ȝm×870ȝm area. With 1.8V power supply, the core circuit of the LPF consumes 1mW and 1.6 mW for WCDMA and GPS/Galileo respectively. The out of band attenuation rate is -80dB/10 octave and the bandwidth error of each mode is less than 4%.