Droplet production is an interesting field of research due to its wide range of applications such as drug delivery and pharmaceutical industry. One of the common methods for droplet formation is using mixture of two immiscible liquids by flow focusing devices. In present proposed device, the dielectric silicon oil and water are used as dispersed and continuous phases, respectively. In addition to characteristics of fluids and flow rates, applying electric field has significant effects on droplet formation and its size. In this study, the effects of various electric fields, flow rates, viscosity ratios and orifice diameters on droplet manipulation by squeezing and dripping regimes and flow focusing method in the prefect dielectric model are investigated. Results showed that at the medium voltages, the bigger droplets were produced by smaller orifices. However, the behavior of the device is reversed at lower and higher voltage ranges. The remarkable output of this paper is introducing a mathematical formula to calculate the droplet diameter as a function of different parameters. This formula, which is derived by numerous numerical simulations, can be used as a tool by designers and researches in this field.