By analyzing the breakage of composites from a new perspective, the tensile strength of composites filled with spherical particles is studied in this article. A large number of experimental data are summed up, and a novel form of formula that can predict the tensile strength of composites is put forward. This form of formula focuses on the composites that possess strong adhesion strength between particles and polymer matrix. High particle volume fraction results in a phenomenon that the particles in composites have a tendency to aggregate. So, the influence of the particle aggregation is also studied in this article. A critical particle volume fraction is proposed to describe that at what value of particle volume fraction the clusters begin to form. And an aggregation factor is put forward to describe the extent that the clusters diminish the composite's tensile strength. A good agreement between the results of the prediction by this article and the experimental data in many references has been found, which validates the accuracy of the theory. POLYM. ENG. SCI., 57:137-143, 2017. (C) 2016 Society of Plastics Engineers