Today, in education many studies have examined the high interest and adoption of social media to assist language teaching and learning activities. Previous researchers have examined the views of teachers' and students' on the use of social media in learning English, but there is no empirical research that examines teachers' belief and students' perception in one study. This study aims to (1) explain students' perception on the use of social media in English class, (2) identify which social media application do students' prefer to learn English, (3) describe teachers' belief on the use of social media in English classes, and (4) analyse gap between students' perception and teachers' belief. This research used a descriptive studies with the qualitative method. And the methods of collecting data in this study included questionnaires and interviews given to collect data from 36 students' of class 10 at MLPB 2 and 3 English teachers' at SMK 2 Negeri Semarang. Then, the data were analyzed using qualitative procedures: data reduction, data display, and data conclusion. This study found that students' perception of using social media helped their learning activities in English class, while the applications they preferred to use were Google and YouTube to help the learning process. Then this research also finds how teachers' become more confident and comfortable when using social media in the learning process. This research also finds a gap between teachers' belief and students' perception regarding how the impact of using social media is both positive and negative. The implication of this research are that students' can better understand their perceptions about using social media, and teachers' trust in social media will be wider, and teachers' should encourage the use of social media and provide more learning spaces using technology to improve students' achievement and educational quality.