By inscribing a long-period fiber grating (LPG) on a polarization-maintaining fiber (PMF), a fiber Sagnac loop sensor for simultaneous measurement of refractive index and temperature has been proposed and demonstrated.The LPG was fabricated on the PMF by using a CO2 laser, and then inserted into a fiber loop formed by using a normal single-mode fiber coupler.One of the transmission minimum of the Sagnac loop sensor was measured, whose wavelength varied with temperature and the intensity changed with refractive index.Temperature sensitivity of -0.654 nm·℃-1 and refractive sensitivity of 49.9 dB·RIU-1 have been achieved.The sensor system shows advantages of small size and low cost, and owns a good application prospect.