A miniature modal interferometer based on a hollow-core photonic crystal fiber (HC-PCF) for refractive index measurement is demonstrated. The modal interferometer is fabricated by splicing the two ends of a 1.2-mm-long HC-PCF to a single-mode fiber (SMF). The air holes of the HC-PCF are fully collapsed by the discharge arc during the splicing procedure, and the length of each collapsed region is about 300 μm. The transmission spectra with different refractive indices outside the HC-PCF are measured. Measurement resolutions of an 8.1×10−4 refractive index unit (RIU) in the range of 1.35 to 1.39, and 4.3×10−4 RIU in the range of 1.39 to 1.43 are achieved, respectively. The temperature effect of the proposed refractometer is also analyzed.