Neural circuits are initially redundant but rearranged through activity-dependent synapse elimination during postnatal development. This process is crucial for shaping mature neural circuits and for proper brain function. At birth, Purkinje cells (PCs) in the cerebellum are innervated by multiple climbing fibers (CFs) with similar synaptic strengths. During postnatal development, a single CF is selectively strengthened in each PC through synaptic competition, the strengthened single CF undergoes translocation to a PC dendrite, and massive elimination of redundant CF synapses follows. To investigate the cellular mechanisms of this activity-dependent synaptic refinement, we generated mice with PC-selective deletion of the Ca(v)2.1 P/Q-type Ca(2+) channel, the major voltage-dependent Ca(2+) channel in PCs. In the PC-selective Ca(v)2.1 knockout mice, Ca(2+) transients induced by spontaneous CF inputs are markedly reduced in PCs in vivo. Not a single but multiple CFs were equally strengthened in each PC from postnatal day 5 (P5) to P8, multiple CFs underwent translocation to PC dendrites, and subsequent synapse elimination until around P12 was severely impaired. Thus, P/Q-type Ca(2+) channels in postsynaptic PCs mediate synaptic competition among multiple CFs and trigger synapse elimination in developing cerebellum.