The effect of the coherence and spectrum distribution of input light on the effective fineness (linewidth) of a fiber optic ring resonator is discussed in this paper. For two typical spectrum distribution, Gauss and Lorentz, the quantitative relation between input light linewidth and the effective linewidth of the ring resonator is given. The analytic results show that the effect of input light with Lorentian spectrum distribution on the effective fineness is greater than that of Gaussian . For Lorentian light source, the effective linewidth is always greater than that of the input light.