Increasing the practicality in mathematics education has been a trend of most countries in the world. With this trend, students find that mathematics is not an abstract subject far removed from reality. One of the pioneers in teaching mathematics to be associated with real-life is Hans Freudenthal - Dutch mathematics educator. He developed the theory of Realistic Mathematics Education (RME) with the idea that teaching mathematics must start a realistic situation; today this theory has been widely applied not only in the Netherlands but also in many parts of the world. In order to approach RME into mathematics education in Vietnam, we developed the RME-based teaching model and conducted research with the teaching of cosine theorem - Geometry 10 to verify its feasibility. The results showed that if the teacher used RME – based teaching, students could learn cosine theorem by abstracting and generalizing from a real situation; thereby, they found that mathematics is very close to life, creating a motivation for learning mathematics for them. From this, we can assert that teaching with the RME approach meets the goal of mathematics education in Vietnam in the present and upcoming period in the 21st century, and it needs to be studied and widely applied in mathematics teaching in Vietnam.