This work interrogates polymer-nanoparticle (NP) composites in electroadhesive (EA) devices to establish relationships among EA force generation and various parameters such as applied voltage, distance, gap between electrodes, dielectric constant, and the surface roughness. High permittivity is especially key for enhancing EA force thus it was modified by inorganic NP-elastomer composite. Fumed SiO2 or Al2O3 NPs were selected as the dielectric material for their homogeneity and permittivity. The NPs were mixed at 1-10 wt% in a polymer resin by using a planetary mixer and subsequently three-roll mill. The distributions of NPs in the composite were established by scanning electron microscopy, energy dispersive x-ray spectroscopy, thermogravimetric analysis, and viscosity measurements. Composites with 10 wt% of Al2O3 showed improvements in mechanical strengths from 1.5 to 2.7 MPa and dielectric constants from 3.4 to 3.6 versus pristine poly(dimethylsiloxane) (PDMS). Flexible EA devices were fabricated by stacking interdigitated electrode patterns and composite films onto a PET substrate, subsequently. An EA device that integrated an Al2O3/PDMS composite with a 10 wt% Al2O3 content achieved force generation as high as 612 mN cm(-2), which is two times higher than the pristine PDMS-coated gripper.