Abstract: A new group-contribution equation of state(VDW-GC EOS)is developed by applying the group-contributionconcept to a local composition version of the modified van der Waals equation of state(VDW-DDLC EOS),which is established by combining the density dependent local composition mixing rule with the modified vander Waals equation of state.Group interaction parameter matrix for thirteen groups were determined frombinary VLE data.The new VDW-GC EOS has been extensively tested on the prediction of vapor-liquid,liquidliquid and gas-solid equilibria.In part Ⅰthe mathematical formulation of the VDW-GC EOS and the determination of the group interaction parameters are discussed.