In web service composition (WSC), quality of service (QoS)-based web services selection has been a critical research issue, and many service selection methods have been presented aiming at resolving this issue, However, most of the existing methods ignore giving a risk evaluation for critical web services in WSC where transactional requirements often require compensation cost to ensure failure atomicity . To address this issue, we present a Risk-driven selection approach by incorporating the impact of failure risk of each participating task to reduce the average losses caused by execution failures of tasks for WSC. With our method, a kind of tree called failure causing tree is built to support risk losses evaluation for each task in composition web service (CWS). Particularly, our method is more suitable to the scientific computing area where many long tasks are often involved and easily lead to a lot of losses such as computing cost, communicating cost, when some failures occur in these executing tasks. The final experiment and evaluation further demonstrate the feasibility and efficiency of our method.