Abstract: The excellent strains were selected in the hybrids from Lilium × formolongi and Oriental hybrid‘Sorbonne’and the hybrid bulblets propagated by tissue culture were used as materials. The anatomy structure of the meristems was observed by paraffin section. The experiment that the materials with different numbers of basal leaves in the short day were moved into the long day preliminarily confirmed the age of photoperiod. Another experiment that the materials in the long day were moved into the short day determined the circumscription of photoperiod. Under different photoperiods，qRT-PCR were used to analysis the expression of CO，FT in different tissues in different growth development stages. The critical periods of the flower induction were determined by the combining of the morphological features，anatomy structure and gene expression. Comprehensively，the hybrids began to enter into the photoperiod competent in the growth development of 6–8 leaves and the stage of flowering induction in the period of 2–4 internodes. Moreover，the circumscription of photoperiod was 28 days.

Key words: lily, photoperiod competent, flower induction, flowering regulatory gene