This paper proposes a voltage-controlled oscillator (VCO)-based 2nd-order continuous-time delta-sigma modulator (CTSDM) for current-sensing readout systems. The proposed VCO-based CTSDM can immediately quantize the current signal from sensor without pre-amplifier. A proportional-integral (PI) structure has been realized by inserting a resistor in series with the integrating capacitor to simplify the circuit complexity as well as maintain the system stability. A noise shaping with second order is implemented by the PI current integrator in the first stage and a VCO phase integrator in the second stage. The complementary current-steering digital-to-analog converter is adopted as the feedback path for the current subtraction. Simulation results show that the proposed current-sensing VCO-based CTSDM can achieve a signal-to-noise-and-distortion ratio (SNDR) of 81.36 dB in 10 kHz bandwidth while consuming only 13.2 μw under 1.2 V supply. This corresponds to a Figure-of-Merit (FoM) of 170.15 dB which is suitable for sensor readout applications in internet of thing (IoT).