Providing guaranteed QoS for Grid services using resource reservation and allocation is an important feature for today?s service Grid. Reservation requests with existing mechanisms are often rejected during the resource utilization peak and lead to resource capacity fragment problem. In this paper, we propose a flexible capacity reservation mechanism, called FIRST, which employs the slack time-enabled request admission control with differentiated selection strategies. We implement the prototype of FIRST in our CROWN node server, the service container of CROWN service grid middleware. The performance of the admission control algorithm for FIRST is evaluated by comprehensive simulations and implementations. Experimental results show that a better resource utilization ratio can be achieved by introducing the slack time into fixed reservation scenario, and a min-min based request selection strategy obtains a better performance compared with existing strategies.