It has been found recently that an ultrasmall nanoparticle whose size is smaller than the thickness of a cell membrane has unique roles in biomedical applications including the development of next generation of drugs or advanced nanoscale cargo carriers.However,the effect of physical properties of an ultrasmall nanoparticle on its adhesion to a bilayer membrane,which is a key step for Nano-Bio interaction as well as the biomedical applications,is still largely unknown.By using molecular dynamics,we find that both size and shape of an ultrasmall nanoparticle strongly affect its adhesion states on a bilayer membrane(e.g.,adhesion,separation or entwined by polymer chains).Interestingly,our simulations show that with decreasing particle size,the effect of particle shape becomes even more evident for the adhesion behavior.It is indicated that the competition between nanoparticle-polymer binding and polymer chain deformation,both of which are influenced by particle size and shape,determines the final adhesion states of an ultrasmall nanoparticle.Our results are helpful for the full understanding of interaction mechanism between nanoparticles and cell membranes and the practical applications of such ultrasmall nanoparticles. It has been found recently that an ultrasmall nanoparticle whose size is smaller than the thickness of a cell membrane has unique roles in biomedical applications including the development of next generation of drugs or advanced nanoscale cargo carriers. However, the effect of physical properties of an ultrasmall nanoparticle on its adhesion to a bilayer membrane, which is a key step for Nano-Bio interaction as well as the biomedical applications, is still substantially unknown. By using molecular dynamics, we find that both both size and shape of an ultrasmall nanoparticle strongly affect its adhesion states on a bilayer membrane (eg, adhesion, separation or entwined by polymer chains) .Interestingly, our simulations show that with decreasing particle size, the effect of particle shape becomes even more evident for the adhesion behavior. It is indicated that the competition between nanoparticle-polymer binding and polymer chain deformation, both of which are are by particle size and shape, determines the final adhesion states of an ultrasmall nanoparticle. Our results are helpful for the full understanding of interaction mechanisms between nanoparticles and cell membranes and the practical applications of such ultrasmall nanoparticles.