The morphologies of flower organs,flowering dynamics,pollen viability,stigma receptivity,pollination methods,out-crossing index,pollen-ovule ratio,pollination and bagging effects were investigated with observation in the outside to determine the breeding system of Tagetes patula.The results showed that heterotypic inflorescence of Tagetes patula was composed of female ray florets and bisexual disk florets.The flowering duration of a single inflorescence was 20-22 d.The flowering process of the inflorescence was classified into four stages including flowering of ray florets,flowering of disk florets,development of achene and maturity of achene.Pollen vitality only sustained 2-3 days after flowering,with the maximum values within the first and second days.The highest stigma receptivity of ray florets was kept from the second day to the fifth day after flowering,while that of the disk florets was kept from the forth day to the fifth day.Insect pollination was the major pollination method and bees and butterflies were the main insects.The combination of the out-crossing index,pollen-ovule ratio and hybridization experiment showed that the breeding system of Tagetes patula was belonged to cross pollination system and pollinators were required.The self-pollination could occur in same inflorescence via the spreading of pollens during the process of stigma elongation.