Taking account of the backscattering in a ring cavity, resonance behavior of two counter-propagating lasing beams is analyzed and the dynamic lasing frequencies as a function of rotation rates are obtained in a close form. The condition for occurring lock-in effect in the gyro is also obtained easily from the vector diagrams. It is noted that the Rayleigh backscattering is the main source to form the lock-in effect, although the scattering is weak in a fiber at infrared wavelength, which can be considerably enhanced in an active resonator. The above mentioned analytic close form of dynamic frequencies is confirmed by a fiber ring laser gyro experiment, and the gyro is locked-in when its rotation rate is less than 10° /s.