Mitofusin-2 (Mtn-2) is a hyperplasia suppressor. Changes in Mfn-2 expression are thought to reflect mitochondrial remodeling during cell proliferation. However, it is unclear how the participation of Mfn-2 in mitochondrial remodeling prevents cellular proliferation. Here we show that arresting vascular smooth muscle cells (VSMCs) in the G(0)/G(1) phase by serum starvation up-regulates Mfn-2 expression and causes mitochondria to assemble into a tubular network and to attach to the endoplasmic reticulunn (ER). In the S phase, short rod-shaped mitochondrial structures that were dissociated from the ER were observed. Levels of glucose, ATP, L-amino acid, and NADP(+) did not vary throughout the cell cycle. However, NAD(+) level was lower and NADH level was higher in the G(0)/G(1) phase than in the S phase. Mitochondrial membrane potential was lower in the S phase than in the G(0)/G(1) phase. Infecting VSMCs with an adenovirus encoding full-length Mfn-2 increased NADH level and reduced NAD(+) level, while infecting the cells with an adenovirus that silences the p21(ras) signature motif produced opposite effects. These results suggest that Mfn-2 up-regulation causes mitochondrial fusion into tubular networks and attachment to the ER, which in turn halts proliferation of VSMCs.