Resource co-allocation is a crucial but challenging problem for Grid Computing. With the emergence of WS-Resource Framework and Open Grid Services Architecture, resource co-allocation is commonly associated with a service level agreement (SLA) to determine the achieved QoS level. In this paper, we present an approach for resource co-allocation in Multi-cluster Grid that maximizes the user satisfaction degree while satisfying the QoS requirements defined in a SLA. The QoS metrics considered in this paper include deadline, cluster availability, service reliability and budget. Experimental results are presented to show the effectiveness of our approach.