Calmodulin (CaM) is an important signal protein in eukaryotes and play multiple regulatory roles in eukaryotes' development in normal and stress conditions. Previous reports showed that calmodulin could accelerate animals and plants' cell proliferation in vitro. However, there is nearly no reports on cell proliferation in vivo in plants under normal and stress conditions. Here, calmodulin antagonist W7 inhibited the root elongation of Arabidopsis seedlings, and the area and cell number of root apical meristem were reduced, the expression of CYCB1;1 gene which plays a key role in cell proliferation was seriously restrained. This data indicates that activated calmodulin is required for cell proliferation. Abscisic acid (ABA), as a key hormone mediating plant adaptation to various environmental challenges, regulates many processes of plant growth and development, such as seed germination and seedling growth. In the present of W7, wild type arabidopsis seedlings are insensitive to ABA, and cam2cam3cam4 triple mutant had decreased sensitvity to ABA. The results implied that CaM might play important role in cell proliferation of root apical meristem and involved in ABA response.