An original design of ring-core photonic crystal fiber (RPCF) is proposed. By splicing a section of the homemade RPCF between two segments of single mode fibers (SMFs), a simple modal interferometer is presented and experimentally demonstrated. Owing to the effects of the collapsed region, the ring modes in RPCF can be effectively activated. To our knowledge, it is the first time to demonstrate the modal interferometer based on the interference between the ring modes, which is different from the previously reported interferometers based on the interference between core modes or cladding modes. The temperature and strain characteristics of the interferometers with different lengths of RPCF are investigated.