A reconfigurable baseband low-pass filter (LPF) with automatic cut-off frequency tuning for WCDMA and GSM application is presented. Cut-off frequencies of 2.1MHz and 200-KHz, and out of band IIP3 at least 20.8dBm and 22.7dBm are achieved under WCDMA and GSM mode respectively. The in band ripple is below 0.5dB in both modes. By utilizing digital frequency tuning technique, a cut-off frequency error of less than 2.5% is obtained. The total tuning time is about 5.5µs. The LPF is fabricated in SMIC 0.13µm CMOS technology with chip area of 0.35mm², and the current consumptions are 8.3mA and 1.1mA for WCDMA mode and GSM mode respectively under a power supply of 1.5V.