A sirocco fan using contra-rotating rotors in which an inner rotor is settled inside the sirocco fan rotor and each rotor rotates in an opposite direction was proposed for the purpose of getting the higher pressure and making the structure of a sirocco fan more compact.If the high discharge pressure is obtained with the adoption of the con- tra-rotating rotors,it could be used for various purposes.Pressure coefficient of a sirocco fan with contra-rotating rotors is 2.5 times as high as the conventional sirocco fan and the maximum efficiency point of contra-rotating rotors shifts to larger flow rate than a conventional sirocco fan.On the other hand,it was clarified from the flow measurement results that circumferential velocity component at the outlet of the outer rotor of contra-rotating ro- tors becomes larger than a conventional one.In the present paper,the performance of a conventional sirocco fan and a sirocco fan with contra-rotating rotors are shown and the internal flow field at the outlet of outer rotor of both cases is clarified.Then,the effect of different kind of contra-rotating rotors on the performance and internal flow field is investigated and the rotor design with higher performance would be discussed. A sirocco fan using contra-rotating rotors in which an inner rotor is settled inside the sirocco fan rotor and each rotor rotor in an opposite direction was was for for purpose of getting higher pressure and making the structure of a sirocco fan more compact.If the high discharge pressure is obtained with the adoption of the con- tra-rotating rotors, it could be used for various purposes. Pressure coefficient of a sirocco fan with contra-rotating rotors is 2.5 times as high as the conventional sirocco fan and the maximum efficiency point of contra-rotating rotors shifts to greater flow rate than a conventional sirocco fan. On the other hand, it was clarified from the flow measurement results that circumferential velocity component at the outlet of the outer rotor of contra-rotating ro-tors becomes larger than a conventional one. In the present paper, the performance of a conventional sirocco fan and a sirocco fan with contra-rotating rotors are shown and the internal flow field at the outlet of outer rotor of both cases is clarified. And, the effect of different kind of contra-rotating rotors on the performance and internal flow field is investigated and the rotor design with higher performance would be discussed.