Micrometric agglomerates of TiO₂ nanoparticles behave as defects, thus lowering AC breakdown strength of the TiO₂/epoxy nanocomposites. However, the mechanism is unclear. TiO₂/epoxy nanocomposites with different agglomerate size were prepared using fabrication process with centrifugation technique. The effects of sample thickness and agglomerate size on AC breakdown strength of TiO₂/epoxy nanocomposite were evaluated in AC breakdown test using McKeown electrode. Assuming that AC breakdown process was avalanche breakdown, the dependence of impact ionization mean free path on the agglomerate size was obtained using 40 generations theory. Interestingly, the mean free path and AC breakdown strength of TiO₂/epoxy nanocomposites had negative correlation to maximum agglomerate size. Probably, reduction of AC breakdown strength of the nanocomposites remaining micrometric agglomerates is attributed to impact ionization accelerated by electric field distortion.