A numerical investigation of the binary collision of two off-center unequal sized liquid droplets in the gaseous environment has been performed. The complete process of collision of droplets is dynamically simulated by solving the incompressible Navier–Stokes equations. Simulations are performed using three-dimensional, multiphase finite volume based solver in OpenFOAM platform. The gas-liquid interface is captured by using the volume of fluid (VOF) method. The solver is appropriately validated and is applied to study the effect of the droplet diameter ratio and the impact parameter on the droplet collision characteristics. The numerical study is done by fixing a moderately high Weber number and Reynolds number for unequal sized droplet collision, where stretching separation is expected as a collision outcome for equal sized droplets. The geometrical characteristics of ligament structures are presented both qualitatively and quantitatively. The satellite droplet formation is also well captured with the present simulations. Post-collision characteristics from the simulations are compared with an existing collision model. It is believed that the present study serves as a first step towards the development of a more accurate droplet collision models. © 2020