Chrysanthemum morifolium is one of the most popular ornamental species worldwide, with high ornamental and economic value. Petal size is an important factor that influences the ornamental value. CmTCP20 is a member of TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTORs (TCPs) gene family, which is closely associated with the growth and development of plants. Our previous study found that the expression of CmTCP20 was obviously down-regulated during chrysanthemum petal elongation, but its function in petal elongation has not yet been revealed. We show here that the overexpression CmTCP20 in Arabidopsis and chrysanthemum leads to similar phenotypes, including larger flower buds (or inflorescences) and longer petals. Interestingly, ectopic expression in Schizosaccharomyces pombe yeast cells showed that CmTCP20 could repress cell division and promote cell elongation. Moreover, the yeast two-hybrid, BiFC and pull-down experimental results indicated that CmTCP20 may regulate petal size via interacting with CmJAZ1-like and inducing down-regulation of CmBPE2 gene expression. This study preliminarily clarifies the function of CmTCP20 on chrysanthemum petal elongation, providing the basic theory for improving the ornamental characteristic of chrysanthemum.