A novel reentrant fiber optical rotation sensor (RE-FORS) is designed and demonstrated. The analytic equations of the optical transmitance, the phase sensitivity and the resolution of RE-FORS were deduced. The contribution to the sensitivity and the resolution of RE-FORS the coupling ratio of the resonator coupler was estimated. According to the theoretical considerations of the sensitivity and the resolution, the setup for investigating RE-FORS with 200-meter-length of ring resonator (RR) was developed. As shown by the test results, the RE-FORS setup has the constant bias of 0.2(°/h) and the variance of random drift rate better than 0.1(°/h) (the integration time of 10 s and the sampling length of 10 min were used).