Abstract The transient coarsening behavior of poly-disperse particles that aggregate into clusters was studied using a phase-field model in two dimensions. The effects of two characters of the cluster, i.e., particle number N _c and particle density ρ _c in a cluster, on the kinetics of transient coarsening are systematically investigated. The transient coarsening experiences two stages: the merging of multiple particles in a cluster into a single particle and further evolution towards the steady state. The evolutions of the scaled particle size distribution and the coarsening rate are non-monotonic during the transient stage. The length of the transient stage for the system with uniform cluster size increases with N _c but is insensitive to ρ _c. Reducing the cluster size homogenization can shorten the transient stage. Moreover, if the coarsening time is scaled by the moment at which multiple particles in all clusters complete the merging, the temporal evolutions of particle radius and particle size distribution are almost uniform for systems with different N _c. This uniform relationship can be used to predict the long transient stage for the system with large N _c, which cannot be directly obtained with limited simulation time.