The effects of round-trip optical length fluctuation (ROLF), assumed to be an ergodic random process, on a fiber-optic ring resonator (FORR) are analyzed in detail. The results show that the effects of small slow ROLF are the same as the temporal coherence of input light, which makes the FORR effective bandwidth to be broadened. But the effect of small rapid ROLF is different, its effect on the effective bandwidth is very small. However if the ROLF is great enough, the FORR will lose its bandpass and bandstop characteristics. These results are verified experimentally.