Using notions of the fractal analysis and models of irreversible aggregation, we present a description of the aggregation process for particles of filler in polymeric composites. It is shown that this process affects fractal characteristics of both particles of the filler and the polymeric matrix. We present quantitative results for distribution of dimensions of these aggregates. We study how variation in structural characteristics affects the properties of dispersed-filled polymeric composites. We propose a simple multi-factor model that clearly illustrates the influence of the aggregation process on the structure and properties of composites.