Web service composition lets developers create appli-cations on top of service-oriented computing and its native de-scription,discovery,and communication capabilities. This paper mainly focuses on the QoS when the concrete composition struc-ture is unknown. A QoS model of service composition is presented based on the fuzzy directed graph theory. According to the model,a recursive algorithm is also described for calculating such kind of QoS. And,the feasibility of this QoS model and the recursive al-gorithm is verified by a case study. The proposed approach en-ables customers to get a possible value of the QoS before they achieve the service. Web service composition lets developers create appli-cations on top of service-oriented computing and its native de -cription, discovery, and communication capabilities. This paper focuses focuses on the QoS when the concrete composition structs-isure is unknown. A QoS model of service composition is presented based on the fuzzy directed graph theory. According to the model, a recursive algorithm is also described for calculating such kind of QoS. And, the feasibility of this QoS model and the recursive al-gorithm is verified by a case study The proposed approach en-ables customers to get a possible value of the QoS before they achieve the service.