This paper presents a impedance-based cytometer for cell anaylsis applications. By adopting a 3-electrode layout and the source-differential sensing scheme, baseline current is cancelled before the sensor front-end circuit to achieve high sensitivity. To leverage the source-differential sensing scheme, a high-gain closed-loop impedance sensing circuit is designed to sense the difference current which corresponds to the difference impedance between the electrodes. The impedance sensor is fabricated in 180 nm CMOS and consumes 9.7 mW at 1.8 V supply with an operating frequency from 50kHz to 40MHz.Measurements demonstrates that the impedance sensor achieves 6pArms input-referred noise over 200 Hz bandwidth at 0.5 MHz modulation frequency. Combined with a microfluidic flow cell, measured cytometry results demonstrate single-cell resolution detecting down to 3μm diameter microbeads in solution with 22 dB SNR.