Flipped classroom, which is also known as “inverted classroom”, is an instructional strategy and a type of blended learning. It reverses the traditional teaching and learning approach by delivering the instructional content, often by video, outside the classroom and filling the in-class with different activities such as discussion. Since it has been coined in the late 1990s, its effect on students' learning outcomes has been fallen into a debate. Inconsistency of the use of in-class activities is one of the main reasons for the difficulties in making comparisons. This study systematically reviewed the current articles (n=12) of flipped classroom in mathematics to investigate the effect of flipped classroom (and their in-class activities adopted) on mathematics learning. Results indicated that the effect of flipped classroom in mathematics is still ambiguous in terms of students' academic performance and perceptions. Further investigation showed that effective flipped classroom, which yielded a better academic result than the traditional approach, always consists of discussion, teachers' feedbacks and peer-collaborative work. A framework of effective flipped classroom in mathematics is then suggested.