Abstract: The hybrid offspring of Rosa chinensis ‘Zhaiye Tengben Yuejihua’×R. chinensis ‘Old Blush’ were used as materials to elucidate the genetic basis of double-petaled flowers in roses. The segregation characteristics of petal number were analyzed. The anatomical observation of flower bud differentiation process of single flower and double flower were performed and the ultrastructure of epidermal cells in petal, stamen and petalized stamen of single flower and double flower were observed. Results showed that the number of petals in the hybrid population was significantly separated and ranged from five to 54. The genetic model of the number of petals, petalized stamens, and pistils was 2MG-AD (two pairs of additive-dominant-epistatic major gene control) and the genetic model of stamen number was 0MG (no major gene control). Double flowers in roses originated as stamens converting into petals. The key period of double flower formation was the late stage of stamen primordium when the stamens changed into petals. Epidermal cell morphology and fold degree of petalized stamens were between the petals of single flowers and the outer-wheel petals of double flowers.