Amine-terminated butadiene acrylonitrile (ATBN) rubber-toughened epoxy networks with different phase separation scale were formulated by adjusting acrylonitrile content in ATBN, and the effect of phase separation on stress-strain behavior, impact strength, dynamic mechanical thermal properties and thermo-gravimetric performance of the toughened epoxy networks were studied in detail. Scanning electron microscope analysis demonstrated that phase separation occurred in the toughened networks, forming a two-phase morphology, and the size of rubber particles was highly dependent on the acrylonitrile content in ATBN. Toughened epoxy network with larger phase separation scale exhibited longer ultimate elongation, and smaller phase separation scale was proved to be more effective in improving impact strength. Furthermore, bimodal rubber-particle distributed epoxy networks were obtained by addition of two types of ATBN simultaneously. The impact strength of the bimodal rubber-particle distributed epoxy network showed a great increase of 47% without sacrificing other mechanical properties, as compared to those of unimodal rubber-particle distributed epoxy networks.