A fiber sensor based on Mach-Zehnder interference (MZI) for simultaneously measuring refractive index and temperature is proposed in this paper.The fiber sensor is constructed by wai st-enlarging a 2cm thin-core single-mode fiber between two conventional sing le-mode fibers.By observing the wavelength shifts of different interference dips and using the sensitivity matrix,refractive index and temperat ure can be measured simultaneously,since the core modes and the cladding modes have different sensitivities to these two external parameters. Experimental results indicate that refractive index and temperature sensitivitie s of the wavelength dips at 1502.54nm are 270.5171nm/RIU and 19.3pm/℃,the refractive index se nsitivity of the wavelength dip at 1521.64nm is 239.5103nm/RIU and whereas the temperature is insensitive.The proposed f iber sensor can reach a resolution of 3.696 6×10－5 RIU for refractive index and 0.5181℃ for tem perature.