The atomic geometry,structure stability,electronic and magnetic properties of VSe2 were systematically investigated based on the density functional theory (DFT).Varying from 3D to 2D four VSe2 structures,bulk 2H-VSe2 and 1T-VSev monolayer H-VSe2 and T-VSe2 are all demonstrated as thermodynamically stable by lattice dynamic calculations.More interestingly,the phase transition temperature is dramatically different due to the lattice size.Finally,owing to different crystal structures,H-VSe2 is scmimetallic whereas T-VSe2 is totally metallic and also they have different magnetic moments.Our main argument is that being exfoliated from bulk to monolayer,2H-VSe2 transforms to T-VSe2,accompanied by both semimetallic-metallic transition and dramatic magnetic moment variation.Our calculations provide a novel structure phase transition and an efficient way to modulate the electronic structure and magnetic moment of layered VSe2,which suggests potential applications as high-performance functional nanomaterial.