Abstract: Resource reservation and allocation is commonly used to provide QoS guarantee for grid services. However, the reservation request with fixed parameters may introduce resource capacity fragment problem caused by the peaks of resource utilization. In this paper, a flexible capacity reservation mechanism is proposed, in which slack time is added into the reservation scenario to get more flexibility of resource reservation. Beside this, an algorithm is introduced to support the reservation and admission control with slack time. Four strategies are proposed according to the existing scheduling algorithms. The algorithm is implemented in CROWN service grid middleware. Evaluation result shows that better performance can be achieved by using slack-enabled algorithm, and the min-min-based strategy performs better.