Synchronizations in coupled limitcycle systems are studied. The order parameter does not change monotonically with increasing coupling. Phase synchronization among oscillators shows a treelike bifurcation, and a number of clustered states are experienced. At the onset of synchronization, simultaneous phase slips of all oscillators and quantized phase shifts are observed. The phase synchronization bifurcation is accompanied by a bifurcation process of the amplitudes of oscillators, that is, the synchronization of phases corresponds to an inhomogeneou sprocess of amplitudes.