Internet of Things (IoT) has gradually become one of the most popular topics among both academia and industry, and it is considered as a crucial part of future Internet. However, very few objective and systematic review was conducted to address high-value articles and summarize the intellectual components from journals for examination and identification of the intellectual core and structure of IoT. Therefore, this study conducted a co-citation analysis for IoT by using 68 high-value articles retrieved within 1457 source papers from Web of Science online database. By using factor analysis, ten critical factors were identified, which includes (1) frameworks and challenges of IoT; (2) current situation of IoT in different applications; (3) interactions of IoT; (4) security issues of IoT; (5) application domains of IoT; (6) data management of IoT; (7) IoT in product lifecycle management; (8) enabling technologies of IoT; (9) IoT in smart cities; (10) IoT in recommender systems. Hierarchical Custer Analysis and Multidimensional Scaling were used to graphically illustrate the intellectual elements of IoT. In the current findings, fundamental elements of IoT including architectural framework, enabling technologies, network communication, data management, and IoT interactions were discussed with a series of challenges for wider and deeper IoT applications.