The lattice Boltzmann method for two-phase fluid flows with large density ratios is applied to the simulations of the collision dynamics of two unequal-size droplets with the diameter ratios of lambda = 0.5 and 0.25 for various Weber numbers of 30 < We < 140 and impact parameters of 0 <= B <= 0.75 at Reynolds numbers of 3900 <= Re <= 4900. The density ratio of the liquid to the gas is fixed at 50. Coalescence collision and two different types of separating collisions, namely reflexive and stretching separations, are simulated. The boundaries between the coalescence collision and both of the separating collisions are found and compared with available theoretical predictions. The mixing processes during separating collisions with lambda = 0.5 for various Weber numbers are also simulated by tracing colored particles embedded in the droplets, and the relation between the mixing rate and the Weber number is obtained. (C) 2008 Elsevier Ltd. All rights reserved.