A gas refractive index sensor with subwavelength-diameter fiber (SDF) based on intermodal interference is designed and its sensitivity is analyzed.The sensor head is formed by splicing a standard single-mode fiber with a stub of SDF without cladding,which only supports the fundamental mode and the 2nd-order mode.The sensor′s sensitivity is analyzed according to the peaks shift of the interference spectrum,which is caused by the refractive index change of the gas outside the sensor head.It is found that the sensitivity of this sensor is higher than the one based on intermodal interference using index-guided photonic crystal fiber (PCF).Since there is no process of gas diffusing into the micro-holes compared with the PCF,therefore this sensor can be used for real-time detecting.