In this paper, a new VCO-assisted VCO-based sigma-delta (ΣΔ) modulator is proposed to improve the linearity of the VCO-based quantizer. The assistant network in the digital feedforward path reduces the input swing of the VCO-based quantizer in the main path, and then adds it together through the digital cancellation path to keep the same signal before and after quantization. Moreover, the merit of the auxiliary VCO increases the tolerance to DAC mismatches because of its intrinsic DEM function, which also simplifies the digital circuit part. A first order continuous-time (CT) ΣΔ modulator with the proposed structure is designed and simulated in a 65nm CMOS process. The performance of the modulator can reach 76.6dB/82.4dB SNDR/SNR with second order noise shaping and 84dB DR within a 10MHz bandwidth and a sampling frequency of 1.4GHz, consuming 9.4mW of power.