This paper sought to examine how behavior of a robot can prompt learning by observing in collaborative learning. The robot learns while solving a problem issued by an English vocabulary learning system with a human learner. The learning system presents English words in example sentences and uses a scaffolding function that helps the learner guess the meaning of English words in the example sentence upon a user request. The robot was designed to solve the questions by using scaffolding function and could not answer correctly at beginning. However, the robot change its question-answering method by guessing the meanings of English words in example sentences and improve its accuracy as learning progressed. This behavior of robot can prompt learners to learn by observing in collaborative learning. Ten college students with low level English learned using the English vocabulary learning system with robot for two months and were videoed during that time to see how they learned. We found that learners learned the English vocabulary by using scaffolding function at beginning. However, learners changed their learning method form using scaffolding function to guessing the meanings of English words in English sentences by learning progress. This suggests that robot, which changes the question-solving method to a more effective one and increases its accuracy rate as learning progress, prompts learners to learning by observing in collaborative learning and change their learning method to the more effective one. This learning by observing indicates that learners learn how to guess the meanings of English words in English sentences by observing the robot’s question-solving and speaking. However, the robot does not prompt some learners to learning by observing because they feel lousy that the robot answers the question and improves its accuracy rate, so they ignore what the robot says. Additionally, learners interest in robot decrease when robot performs the same action.