The coalescence of core-shell type polymer microspheres was investigated through electron microscopy. The core and shell were formed by crosslinked poly(2-vinylpyridine) (P2VP) and polystyrene (PS) multi-branchings, respectively. The microspheres were dispersed in organic solvents. The arm length of PS branching was controlled by ozonolysis at the position of the introduced isoprene units. The aggregation of microspheres depended strongly on the volume fraction of shell portions. In microspheres with an extremely thin shell, coalescence between P2VP cores was caused by poor steric stabilizing of PS branchings in the dry film.