Abstract. This paper contains several results concerning circle action on almost-complex and smooth manifolds. More precisely, we show that, for an almost-complex manifold M 2mn (resp. a smooth manifold N 4mn ), if there exists a partition λ = (λ1, · · · , λu) of weight) is nonzero, then any circle action on M 2mn (resp. N 4mn ) has at least n + 1 fixed points. When an even-dimensional smooth manifold N 2n admits a semi-free action with isolated fixed points, we show that N 2n bounds, which generalizes a well-known fact in the free case. We also provide a topological obstruction, in terms of the first Chern class, to the existence of semi-free circle action with nonempty isolated fixed points on almost-complex manifolds. The main ingredients of our proofs are Bott's residue formula and rigidity theorem.