The interactions between droplets have an important influence on the atomization of liquid fuel, the combustion efficiency, and the reduction of particulate matter emissions for an engine. For this reason, this paper presents results from an experimental study on the coalescence and break-up of droplets after collision. According to the shape and parameters of the droplets at different times after the collision of the droplets was captured by a high speed camera, analysis was done for the following effects of droplet collisions: the collision-coalescence motion for the collision between the droplets, the change history of the dimensionless length-to-width ratio of the oscillation motion, the critical size ratio of the breakup motion, and the liquid physical properties of the particles. The results show that the droplets collide and exhibit two forms of coalescence oscillation and break-up: for oscillating motion, at higher droplet collision velocities and dimensionless size ratios, there will be a larger dimensionless length-to-width ratio for the droplet oscillation; for the break-up motion, at higher collision velocities, there will be lower dimensionless size ratios, and lower liquid surface tension, shorter times over which the droplet breaks, and facilitated droplet break-up. The research results presented here can be used for atomization in engine cylinder, increasing the gas/liquid contact area and enhancing the combustion efficiency of gas/liquid heat transfer to improve the combustion efficiency of the engine. (C) 2019 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.