An H-function in analogy with the entropy in thermodynamics is proposed for intra-molecular vibrational energy redistribution (IVR) in an algebraic approach, which is based on an algebraic classical Hamiltonian on the coset sphere where actions are treated as continuous variables. The time rate of the H-function serves nicely as a tool for IVR and is applied to show the resonances in H2O and DCN in a compact way.