Although many researchers have pointed out that educational robots can motivate student learning, learning motivation inevitably declines over time. The sustainability of learning motivation is closely related to instructional strategies. In other words, appropriate instructional strategies are still essential to sustain a learner's learning motivation in robot-assisted instructions. The ARCS model provides systematic guidelines for enhancing and sustaining learning motivation; however, it provides very limited instructional practices in the model. Recent development in educational robot grows rapidly. A humanoid robot, which has a tangible and attractive body and limbs, is able to perform movements and gestures and to interact with its users. This study aims to develop instructional strategies and activities based on the ARCS model and the specific features of the robot teaching assistant (RTA) to enhance and sustain motivation in learning English reading skills. A quasi experiment and a survey were conducted to evaluate the effects of the designed strategies and activities. The results showed that the design of the RTA-based activities following the ARCS model for learning English reading skills was positively and significantly contributed to students¡¦ learning motivation, learning performance and continuance intention.