The transition from vegetative growth to flowering is a major developmental switch in the plant cycle and the timing of flowering is very critical for reproduction of plant species. In transition to flowering in plants, Flowering locus C (FLC) is one of the crucial factors. Here, we showed How the stability and activity of FLC are regulated by sumoylation mechanism. By pull-down assay, we showed that FLC interact with E3 SUMO ligase in vitro and vivo. And we showed that FLC is sumoylated in vitro condition with AtSUMO1 protein. In transgenic plants with overexpression of FLC and inducible expression of AtSIZ1, sumo E3 ligase led to increase of FLC protein level and delayed the post-translation degradation of FLC indicating that Arabidopsis E3 sumo ligase AtSIZ1 stabilizes FLC. Also, the plants with overexpression of mutant FLC (K154R, a mutation of the sumoylation site on FLC) flowered considerably earlier than plants with overexpression of FLC but comparable with wild type indicating that sumoylation is a important part for function of FLC. Our data indicate that the sumoylation of FLC is critical for its role in the control of flowering time.