Artificial intelligence (AI) has gained importance and popularity in education recently. As an AI-based chatbot, ChatGPT has attracted the attention of educators and researchers from different fields, as well. The present study aims to explore the effect of ChatGPT-integrated English lessons on the 10th grade Turkish EFL learners' writing skill and vocabulary development. Additionally, it attempts to gather the perceptions of the participant students about ChatGPT-integrated language teaching. This research is a mixed-method study which combines both quantitative and qualitative data. 66 students who study at two different classes participated in the study by taking place in experimental phase and completing pre- and post-tests. 7 students took place in semi-structured interview at the end of the implementation. Throughout the implementation, the experimental group took ChatGPT-integrated vocabulary and writing instruction whereas the control group took traditional instruction which lasted for 5 weeks. Quantitative data were calculated statistically by using SPSS 29 and qualitative data were investigated by conducting thematic analysis via MaxQDA24. The results of pre- and post-tests showed that the traditional instruction had more effect on writing and vocabulary development of the participant students. The findings gained from semi-structured interviews revealed that students consider mostly positively ChatGPT and they reflected some concerns about academic integrity and ethics and laziness in learning. There are some pedagogical implications recommended for teachers, students and researchers at the end of the study.