Morphology of floral differentiation on the top of tobacco stalk was investigated with stereomicroscope, and pot and field experiments were conducted with the observation on the button stage and early flower period to compare the sensitivity to low temperature of six flue-cured tobacco cultivars. The results showed that observation on floral differentiation with stereomicroscope was in accordance with the button stage in pot and field experiment. In terms of the loss of available leaves and the floral differentiation, Yunyan85 and Yunyan201 were considered less sensitive than Yunyan97 and Yunyan87, but quite more sensitive than Hongda. While the growth and development of K326 was prolonged in foral differentiation, button stage and flowering.