The main purpose of this study is to develop a conceptual understanding of the irrational number of the square root of 2 ([square root]2 ). Participants in the study were 20 ninth-grade male students. Activity Theory was used as a framework to show the development of the conceptual understanding. Since this study was conducted during the COVID-19 pandemic; online teaching method was adopted. In this teaching method, WhatsApp messaging and calculator were used as our basic technology. Virtual education lasted 2 sessions (120 minutes) for the development conceptual understanding of the irrational number of square root of 2. To produce data, WhatsApp Export Capability was used. For data analysis, the online teaching activity system was used. By analyzing this activity system, three tensions were understood. Modifying these tensions, has led to their students making the concept of the irrational number of the square root of 2 ([square root]2 ) and reach a single definition.