In grid computing and commercial application environment, it is an important link to protect user QoS requirements, and optimize service efficiency and effectiveness of system resource. For ensuring the balance of QoS requirements between two sides of supply and demand, a method of estimating the QoS expectation for users is proposed in the paper, which can compute a set of balanced QoS parameters value in view of the QoS ability of provider based on the principle of effectiveness. The service selection can obtain a higher efficiency of utilizing by using the proposed method. An actual example with analysis shows that it could reliably estimate a set of effective QoS expectations.