Abstract: In this paper, the influence of the lower pressure limit of the instrument on the efficiency of the mercury withdrawn under low expulsion pressure is discussed, based on the experimental data combined with theoretical analysis. Thereafter, the author points out the problems in the measurement of size distribution of pore throats using mercury withdrawn curves and proposes a new method of mercury repressure curves to determine the size distribuion of pore throats. Finally, a simplified method of "sequentially reducing pressure mercury withdrawn curves" is presented.