The previous researches results seem to suggest that some aspects of learning of diffusion and osmosis concepts such as membranes, kinetic energy of matter, and elements of the particulate and random nature of matter could lead to misconceptions. The concept of diffusion is very common in science instruction, and understanding the concept is an important precursor to instruction in life science and physical science. The learning of concentration and tonicity processes of diffusion and osmosis, life forces influence on diffusion and osmosis, and random nature of matter were difficult learning for students. It appears that misconceptions may play a larger role students learn diffusion and osmosis concepts. What could be take place the active role of the student learning without misconceptions of diffusion and osmosis? It is possible suggest, students should actively engaged in constructing knowledge. During each phase of the learning, students should actively manipulating materials, recording data, or analyzing results. Students should encourage discussing findings in groups and with the class. The teacher acts as a facilitator via making connections between concepts. Students may debate and argue relationships between concepts and their contents this may provides paying their attention experiences with the concepts. Because of its importance of diffusion and osmosis, it may be beneficial to investigate misconceptions of high school students. The purpose of this study was to investigate students' understanding about scientifically acceptable content knowledge by exploring the relationship between knowledge of diffusion and osmosis and a student's confidence in their content knowledge following instruction.