The off-center collision of binary equal-sized alumina droplets has been investigated by employing a three-dimensional direct numerical simulation method which involves the volume of fluid (VOF) and adaptive mesh refinement method. The simulations of tetradecane droplet collision are carried out to validate the numerical method. The results show good agreement with experiments. The off-center collisions of alumina droplet with diameter 10 mu m are numerically investigated at various Weber numbers (30-400) and impact numbers (0.1-0.7). The Ohnesorge number is 0.1151. Four collision outcomes are obtained, coalescence after substantial deformation, reflexive separation, coalescence after long extension, and stretching separation, respectively. Bouncing is achieved by employing the double VOF functions method. In addition, as to the collision at various impact parameters, the critical Weber number of separation is obtained. And the collision regimes of alumina droplet are mapped out. Furthermore, alumina droplet collisions are modeled with the consideration of viscous dissipation, including the model of coalescence after minor deformation, bouncing model, reflexive separation model, stretching separation model, as well as the model for calculating the droplet parameters after separation. Bouncing model, reflexive separation model and stretching separation model are in good agreement with the present numerical results. (C) 2017 Elsevier Ltd. All rights reserved.