An optical voltage transducer for the electric power system based on modular interference in dual-mode two-hole photonic crystal fiber (PCF) is designed. A quartz crystal cylinder wrapped with two-hole photonic crystal fiber is used as the voltage sensor head. The intensity of the two interference output lobes of the two-hole photonic crystal fiber is modulated with the converse piezoelectric effect of quartz crystal. In low voltage part, a piezoelectric ceramics (PZT) which wrapped with a piece of double core PCF is used for adjusting the static phase difference at orthogonal status. The experimental results show that the measurement precision is 0.5% by rated voltage in laboratory conditions.