There have been few reports on the morphology of flower opening, despite its horticultural significance. It is not clear when cell division stops during rose petal development or what changes occur in cell morphology. This study aims to clarify the details of cell morphological changes during rose petal development. Rose (Rosa hybrida L. 'Sonia') petals were sampled in six flower bud stages. Cell morphological changes were observed by light microscopy, transmission and scanning electron microscopy using cross sections of the petals, and the number of epidermal cells was measured using Nomarski differential interference contrast microscopy. The number of epidermal cells increased with flower opening, but the rate of increase in the number of abaxial epidermal cells slowed down at an earlier stage than in adaxial epidermal cells. The increase in the epidermal cell area was much more rapid in later stages compared with the increase in cell number, suggesting that petal growth in later stages is mainly due to cell expansion. During flower opening, the unique expansion of spongy parenchyma cells produced large air spaces. Epidermal cells of the tipper part showed obvious lateral expansion. In particular, marked expansion of adaxial epidermal cells with enlargement of the central vacuole was observed. Differences in the patterns of cell expansion among cell types and locations would contribute to the reflex of petals during rose flower opening.