Abstract: Delayed flowering of chrysanthemum under high temperature conditions is a serious obstacle for all year round cut chrysanthemum flower production in southern temperate and subtropical zones. To clarify the causes of flowering delay in spray chrysanthemum, two different genotypes of spray chrysanthemum (Dendranthema grandiflorum (Ramat.) Kitam. syn. Chrysanthemum morifolium) were grown under high-temperature conditions: summer-to-autumn flowering type (SA type, high temperature tolerant) and autumn flowering type (A type, high temperature sensitive). Their flower-bud initiation and development were subsequently compared. Results clarify that two independent events caused by high temperatures occur in the shoot apex of spray chrysanthemum under short-day conditions. First, high temperatures slowed floral development in inflorescence, thereby increasing the number of florets in both SA and A chrysanthemum genotypes. Secondly, high temperatures slowed the developmental speed of inflorescence after the budding stage, and the time to reach the bud break stage was prolonged, thereby delaying flowering, especially in A chrysanthemum genotypes.