The purpose of the present study was to construct the theoretical dissolution model of poly-disperse drug particles in biorelevant media containing bile salt/ lecithin aggregates (micelles or vesicles). The effective diffusion coefficient in the biorelevant medium and the particle size distribution of drug particles were simultaneously factored into the Nernst-Brunner equation. The effective diffusion coefficient of a drug in the biorelevant medium was calculated to be smaller than that in the blank buffer, since the diffusion coefficient of a drug bound to the aggregates became similar to that of the aggregates. The particle size distribution of a drug powder was simulated as the sum of mono-disperse fractions covering the particle size range. To verify the modified equation, the dissolution profile of griseofulvin and danazol in a taurocholic acid/egg lecithin (4:1 mixture, taurocholic acid ΒΌ 0-30 mM) system was investigated. It was clearly demonstrated that both modifications on the Nernst-Brunner equation improved the prediction accuracy. When the effect of the particle size distribution was neglected, the theoretical curve underestimated the observed value at the early phase of dissolution process. When the diffusion coefficient of a free drug was used instead of the effective diffusion coefficient, the theoretical curve overestimated the observed value. The results of the present study suggested that the effect of the particle size distribution and the effective diffusion coefficient should be taken into consideration.