Although many researchers have pointed out that educational robots can stimulate learners' learning motivation, the learning motivation will be hardly sustained and rapidly decreased over time if the amusement and interaction merely come from the new technology itself without incorporating instructional strategies. Many researchers have identified that the sustainability of the learning motivation is closely related to the instructional strategies. Accordingly, learners must be well guided by suitable instructional strategies to enhance and sustain their learning motivation. In other words, suitable instructional strategies are required even if introducing educational robots to learners can stimulate learning motivation at the beginning of learning activities. Literature also suggests that systematic instructional strategies can be designed with the use of the Attention, Relevance, Confidence, and Satisfaction (ARCS) model. As the rapid development of robotics, it has become feasible to apply the educational robot for enhancing learning. A robot teaching assistant (RTA) was designed to enhance and sustain learning motivation for the learning of English reading skills. A quasi-experimental design and the Instructional Material Motivational Survey questionnaire were used for data collection and evaluation. The findings showed that the design of the RTA-based learning activities guided by the ARCS model for learning English reading skills can significantly improve learners' learning motivation, learning performance, and continuance intention. (Contains 9 figures and 7 tables.)