The information on grain size of reservoirs from gamma logging curves can be obtained and the grain size parameter can be extracted based on the dyadic wavelet transform. Considering the influence of sampling intervals caused by the measurement rate of logging tools, an algorithm for solving the system of recurrence equations concerning volume fraction in different kinds of grain size distributions was presented through analyzing the content by volume in different kinds of grain size distribution and the relationship between range and frequency of signal which was extracted after gamma logging curves changed. On the basis of this algorithm, the absolute volume content in different kinds of grain size distribution can be transformed into the relative volume fraction.