This paper presents a fully-integrated low-pass R-MOSFET-C filter with a tunable cutoff frequency basing on a typical topology of the active RC filter. A MOSFET is employed in the proposed filter to replace the resistor in the original topology to achieve the tunable and ultra-low cutoff frequency, which is suitable for biomedical applications. The filter is implemented in UMC 0.13um CMOS technology with a 1.2V supply voltage. The simulation results show that the cutoff frequency ranges from 0.2Hz to 292.8Hz as the gate voltage of the MOSFET is tuned from 0.6V to 0.84V.