Abstract: Flowering state heat hurt degree and relative physiological index were measured to determine the heat tolerance of five new cultivars in Dendrthema with small flowers. Results showed that flower bud differentionation developing and flower colour weren，t affected by high temperature in heat tolerant cultivar, which showed late and least fall in chlorophyll a、a＋b content,slowest rise in eletric conductivity with the prolongation heat treatment time. Ai and A8 were the best heat tolerant cultivars,while A9、Y were the worst heat tolrant cultivars,A27 was arranged between them.