A conventional technique for microfluidic droplet generation is Co-axial Flow Focusing (CFF) in which a contraction zone is placed downstream of the dispersed phase nozzle. In this contraction zone, the dispersed-phase (d-phase) fluid is pinched off by continuous-phase (c-phase) fluid to generate micro-droplets. Studying the influence of multiple parameters such as the fluids velocities and viscosities, the interfacial tension, and nozzle and orifice diameters on the droplet size is of great importance for the design and application of CFF devices. Thus, development of more complete numerical models is required. In this paper, we show our model is compatible with experimental data and then numerically investigate the effects of aforementioned parameters on the droplet generation in a CFF microfluidic device. Simulation results showed that the c-phase flow rate, viscosity and the interfacial tension had great impacts on the droplet size. The elect of the nozzle diameter on the generated droplet size was small compared to that of the orifice in a CFF device. Using the simulation results, a correlation was also developed and suggested which predicts the droplet size with less than 15% error in a wide range of the introduced dimensionless parameters. (C) 2020 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.