An analog front-end (AFE) used in portable electrocardiogram (ECG) monitoring devices is proposed. This AFE has included all necessary functions for the commercial applications. The core circuit consists of the instrumentation amplifier (IA), a 2(nd) order Butterworth low pass filter, and the second amplifying stage. The driven-right-leg circuit is integrated in the IA to effectively suppress the common mode interference. And the power management circuits provide a stable supply voltage, bias current and reference voltage for the other circuits. To guarantee the validity of the continuous monitoring data, the leadoff monitoring circuit is developed to monitor the connection of the leads. The chip is taped out with SMIC 0.18 µm CMOS process, and the measured results show that the common mode rejection ratio (CMRR) and power supply rejection ratio (PSRR) achieve 75 dB and 90dB respectively, and the equivalent input referred noise is 12 µV.