Social media is a mass communication tool that emerged with Web 2.0 technology and allows users to not only be buyers but also produce content. Instagram is the second most preferred social media tool by users. As it offers good options for photo and video editing, enables users to interact with each other easily, acts as a bridge by including the features of other social media tools, the fact that the average age of users is young, and its accessibility is facilitated by the widespread use of mobile phones, has made Instagram one of the most preferred social media tools by secondary school age group. The aim of this study is to detect the idioms that are used in Instagram posts. In this research, 84 contents from 14 different categories that are followed by the students are analyzed. In the study, which has a descriptive nature in the screening model, the contents are evaluated with the systematic analysis approach. In the end of the research, we detected 95 idioms from 50 contents out of 84. While the usage of 45 idioms out of 95 is true, the other 50 idioms are used wrong with syntax errors and with changes that will break the pattern of the idiom. Key Words: Social Media, Instagram, Idioms