As an important part of Cloud Manufacturing (CMfg), Industrial Cloud Robotics (ICRs) encapsulates manufacturing capability of physical industrial robots as services for the users. However, a growing number of functionally equivalent services appear in CMfg platform due to the wide use of industrial robots in manufacturing field. It is important to carry out Manufacturing Capability Service (MCS) optimal selection for ICRs from various optional services under CMfg environment. But current service optimal selection method emphasizes on the non-function information of services, and it ignores the interactive relationships between different services and the basic function information of services, which make it difficult to satisfy the various personalized demands of users. Service optimal selection requires the integration and sharing of manufacturing knowledge. Knowledge graph provides an effective way to express and manage knowledge. And it can provide decision support for users to select appropriate ICRs service. Therefore, this paper proposes a method of knowledge graph-based manufacturing capability service optimal selection for ICRs. The function information, association information and non-function information of MCS are described based on knowledge graph. Based on this, the service optimal selection procedure is proposed to realize smart MCS optimal selection for ICRs, which includes feature selection, association selection and user custom weights of non-function indices selection. Finally, a case study based on robotic assembly is presented to demonstrate the effectiveness of proposed method.