In this talk I will explain the observation that Nekrasov's instanton partition function, which is a certain equivariant integral over the framed instanton moduli, gives the conformal block of W-algebras, including Virasoro algebra. This strongly suggests that the equivariant cohomology of the instanton moduli has the structure of Verma module of W-algebras. This is based on my joint work with L. F. Alday and D. Gaiotto.