Abstract: The fiber fine structure in solar radio bursts carries small-scale magnetic field information in the initial phase of the burst.We deal with fiber structures to analyze the frequency drift rate.The background is obtained by a wavelet transformation.With a subtraction of the background and a threshold processing,the fiber structures are seperated.We select continuous segments in each channel.The cubic spline interpolation is used to fit the intensity-time relation with which the moments of intensity maxima are determined.Finally,the drift rate is calculated with linear regression.The algorithm is used to calculate frequency drift rate of the radio fiber event on 21-April-2002.The mean frequency drift rate obtained is beween -0.041 0~-0.013 8 GHz·s^-1.