Network Functions Virtualization (NFV) offers a new way to design, deploy and manage network services. NFV relocates network functions from dedicated hardware appliances to generic servers, so they can run in software. Also, it is designed to consolidate and deliver the network components needed to support a virtualized infrastructure including virtual servers and storage, as well as networks. However, incomplete or inconsistent configuration of virtualized network functions (VNF) and forwarding graph (FG, aka service chain) could cause breakdown of the supporting infrastructure. Moreover, automation of NFV process with integration of Software-Defined Networking (SDN) technologies may lead the network services to be more error-prone. Recognizing these problems, we need to identify key properties to be checked. In this article, we present a framework of verification for NFV-enabled network services. Its goal is to provide a formal method for the network services that can be used to build and verify NFV components. Overall, this method makes it safer and more reliable for network operators to compose their network services in NFV environment, so that they can be assured that the properties are correctly enforced.