A fiber-optic sensor for simultaneous measurement of refractive index (RI) and temperature was proposed and demonstrated. It was fabricated by cascading two sections of specialty double cladding (DC) fibers which presented a pair of well-separated resonant spectra dips. The sensing properties of temperature and ambient RI were investigated theoretically based on the coupled mode theory. Experimental results indicated that these two resonant spectra shifts were linearly dependent on the variation of the RI in the range of 1.3333 approximately 1.4118 and on the temperature in the range of -10 degrees C approximately + 80 degrees C. Such a fiber-optic sensor is simple and easy for mass production and has potential applications for biosensors or chemical sensors.