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In this paper we investigate the existence of model-equivalence reduction between NP-logic systems which are logic systems with model existence in NP.It is shown that among all NP-systems with model checking problem in NP,the existentially quantified propositional logic((∈)PF)is maximal with respect to poly-time model-equivalent reduction.However,(∈)PF seems not a maximal NP-system in general because there exits an NP-system with model checking problem DP-complete.