This paper describes effect of characteristics of dielectric layer on performances of droplet transport in EWOD (Electro wetting on dielectric) microfluidic devices. EWOD is a technology for manipulating droplets discretely by controlling the surface wettability electrically. Films of Parylene C, PDMS, and SU-8 were prepared for the dielectric layer and effects of deposition method, thickness and permittivity of these films were investigated experimentally. The results show that water droplets can be easily transported at low voltage when the dielectric film with high permittivity is deposited thinly and uniformly.