Interaction is key in English language learning. According to the interaction hypothesis, more active and appropriate interactions are important for effective learning to occur. Focusing on writing skills learning with this perspective, the purpose of this study was to analyze the effects of interaction on writing skills at the elementary English level. This study devised collaborative writing activities that can promote interaction, and applied them to writing learning in the 5th and 6th grades. The effects of collaborative writing activities were examined in terms of interaction (input, output, feedback, and modified output), writing skills (fluency and accuracy), and perception of writing learning (interest, confidence, participation, importance, and usefulness). The main results are as follows: first, collaborative activities helped to promote interaction, and interaction took place in a way that was helpful for writing learning. Second, collaborative activities helped to improve fluency and accuracy in writing skills; this may be because students in collaborative activities can use diverse prior knowledge. Third, collaborative activities helped to change the perception of writing learning positively. These findings are expected to provide important insights needed in planning writing lessons with an emphasis on interaction