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Abstract. In this paper, we introduce the notion of generalized rational Okamoto-Painlevé pair (S, Y ) by generalizing the notion of the spaces of initial conditions of Painlevé equations. After classifying those pairs, we will establish an algebro-geometric approach to derive the Painlevé differential equations from the deformation of Okamoto-Painlevé pairs by using the local cohomology groups. Moreover the reason why the Painlevé equations can be written in Hamiltonian systems is clarified by means of the holomorphic symplectic structure on S − Y . Hamiltonian structures for Okamoto-Painlevé pairs of typeẼ 7 (= P II ) andD 8 (= PD 8 III ) are calculated explicitly as examples of our theory.