Network function virtualization enables a flexible service provision by introducing the concept of virtual network function (VNF). Despite the importance of VNF scheduling, it is largely unexplored. In this letter, we propose a new model for VNF scheduling based on the min-plus algebra theory. The main idea of this model is to share the deployed VNF instances among different services, thus to improve the resource utilization and reduce the resource fragmentation generated by deploying many VNF instances. In addition, a weight-based VNF sharing scheduling approach is proposed in this model to achieve a fair scheduling among the arriving service requests. The experimental results show that the proposed model and approach are effective and efficient.