Estimation of particle penetration through HEPA (High Efficiency Particulate Air) filters with the conventional filtration theory requires accurate measurements of average fiber size and variance of fiber diameters. However, it is not an easy task to obtain these properties because fiber size distribution varies to a large extent from point to point in a HEPA filter. In the present work, measurements of representative fiber size distribution from scanning electron micrographs of HEPA filter cross-section are performed and the particle penetrations are predicted with the measured fiber size distribution. They are compared with those from the conventional method (measurement from filter surfaces) and experimental data. As a result, even for filters with inhomogeneity factor delta(p)<1 by the conventional method, the present method gave delta(p)>1, suggesting that delta(p)<1 is caused by the failure in measurement for representative fiber size distribution. However, the improvement in prediction with more reliable filter properties is not satisfactory because the conventional filtration theory cannot account for the contribution of fine fibers properly. Introduction of bimodal fiber size distribution together with the measurement of fiber size distribution in a cross-section of a filter is found to give comparable prediction results with the conventional method without resorting the inhomogeneity factor of filter packing.