Enthalpy economizer can theoretically save more energy than temperature based economizer. However, the requirement of outdoor air humidity measurement in the traditional enthalpy economizer control made it impossible. A novel control sequence using mixed air enthalpy is developed in the paper. Both theoretical and experimental investigation shows that humidity measurement in mixed air duct is very reliable, and the proposed method can achieve true enthalpy economizer saving. A case implementation shows 15.7% more energy saving than temperature based economizer in Omaha, NE.