Droplet generation in hydrodynamic flow-focusing microfluidics device are numerically studied by solving the time-dependent Navier-Stokes equations of two-phase flow using volume-of fluid (VOF) method and level set method. We present the results regarding the production mode of single, double and multiple droplet sizes. The numerical results are in consistent with the experiments, and different factors, such as Reynolds number and Webber number, influencing the dynamics of droplets breakup were also studied.