The transition to phase synchronization in systems consisting of a large nu
mber (N) of coupled nonlinear oscillators via the route of phase clustering
(phase synchronization among subsets of oscillators) is investigated. We e
lucidate the mechanism for the merger of phase clusters and find an algebra
ic scaling between the critical coupling parameter required for phase synch
ronization and N. Our result implies that, in realistic situations, phase c
lustering may be more prevalent than full phase synchronization.