A simple modal interferometer is proposed, to be produced by splicing a section of a novel ring-core photonic crystal fiber (RPCF) and two segments of single-mode fibers. Owing to the effects of the collapsed region, the ring modes in the RPCF can be effectively activated. This is the first demonstration of an interferometer based on the interference between ring modes, as opposed to previously reported interferometer based on the interference of core modes or cladding modes. The effects of the length of RPCF and the wavelength on the temperature and strain characteristics of the interferometer are investigated. The strain sensitivity increases with RPCF length and with wavelength. Based on the interferometer, a strain sensor with low temperature sensitivity is proposed, which has strain sensitivity of 1.777 pm/(µm/m).