During the follicular development process, granulosa cells proliferate and then differentiate to acquire the ability to respond to ovulation stimuli. It is known that FSH activates cAMP-dependent signaling pathways which induce target gene expressions in granulosa cells. However, there have been few reports about the ATP production pathways which induce cell proliferation in granulosa cells. Moreover, the reasons why an expansion of the diameter of antral follicles is required for granulosa cell differentiation remain unclear. This review introduces our current research showing that the transcription and translation of mitochondria are activated in FSH-stimulated granulosa cells, and this has an impact on ATP production, which in turn induces cell proliferation. Granulosa cell proliferation is essential for changes in the DNA methylation status, especially in the Lhcgr promoter region.