Nano-clay and micro-silica mixed composites were made for use as insulating materials for switchgear. Observation of nano-clay dispersion and measurements of insulation breakdown properties under a homogeneous electric field were conducted of these composites. Moreover, simulations were carried out to estimate the curing reaction and rheological behavior of the composite in an automatic pressure gelation system. Experimental results demonstrated that these composites have suitable material characteristics for practical use in switchgear.