We have proposed a refractive index sensor, which is based on dual-core photonic crystal fiber (PCF) with a rectangular air-hole lattice structure. The two fiber cores of the PCF are separated by single air-hole filled with the analyte liquid and the refractive index of the analyte liquid (na) varies from 1.33 to 1.41. The highest sensitivity of 9,400 nm/RIU (Refractive index unit) can be acheived when na is 1.41 in the simulated results and lowest sensitivity is up to 5,940 nm/RIU when na is 1.33.