A respiratory monitoring system was proposed based on optical fiber. The sensor consists of photonic crystal fiber (PCF) spliced with multimode fiber (MMF) to fabricate Michelson interferometer. The sensor is inserted into an oxygen mask tube and then put on the nose and mouth to monitor the respiratory rate (RR) for different cases. The results showed that when breathing increased, the intensity of light decreased. The intensity of light in the sleeping case of breathing was more intense than that when the intensity of working sport was measured. The working sport case was a higher transmittance than that of sleeping. The sensitivity of the sensor was also measured to be 231.9 pm/RH.