Subwavelength and nanometer diameter optical fibers have the optical property of enhanced evanescent fields, which makes them very sensitive to the index change of the ambient medium. In this paper, a novel refractometric sensor based on fiber Bragg grating in micro/nano-fiber (MNFBG) is proposed, integrating the enhanced evanescent fields of micro/nano-fiber (MNF) with wavelength choice feature of FBG, and the fabrication about MNFBG is discussed. Refractive index sensing characteristics is comprehensively investigated in theories, and the simulation of sensing the variation of ambient refractive index is performed by using the software of OptiGrating. Results show the sensitivity of the MNFBG is increasing with the decrease of the radium of the MNF. MNFBG with the radius of 400nm can achieve the sensitivity as high as 993nm/RIU. This value is enhanced by 170 times over that of the FBG with etched cladding, which demonstrates that MNFBG has great potential of application in miniaturized, high-sensitivity refractometric sensors.