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In order to improve the rheological behavior of nanosilica composite no-flow underfill, a filler surface treatment using silane coupling agents was investigated, to reduce the filler-filler interaction and to achieve monodispersity of the nanosilica in the underfill resin. The experimental conditions of the surface treatment were investigated in a DOE in terms of the pre-treatment methods, coupling agent types, concentrations, and treatment durations. The particle dispersion after treatment was evaluated by a laser particle analyzer and TEM. A mono-dispersed nanosilica solution in the polar medium was achieved using optimal experimental conditions. The surface chemistry of the nanosilica was studied using FTIR. The wettability of the underfill resin and water on the silane treated glass slides was studied using a goniometer. Based on the investigations, the silane-treated nanosilica fillers were incorporated into an underfill resin to formulate a nanocomposite no-flow underfill. It was found that a proper filler treatment could significantly reduce the viscosity of the nanocomposite.