Constructivist suggest that teachers should guide and facilitate their students in reinventing and or reconstructing mathematics concepts as in reconstructing a theorem. A theorem in mathematics hold an important role as a fundamental aspect in building the mathematics itself. Hence, it is necessity for the students to have and learn this ability. Furthermore, by reconstructing a theorem, students not only learn about the theorem, but also learn about problem solving since the theorem that should be reconstructed can be presented as a problem. In which, this skill is needed and promoted in the 21^st century. Since teacher should be able to guide and facilitate their student in reconstructing a mathematical theorem, the teacher has to be able to reconstruct and proof their construction of theorem. Yet, many researches were focused on investigating students’ ability in proofing a mathematical not on the ability of reconstructing a theorem. the This study is aimed to investigate mathematics undergraduate students’ ability in reconstructing and proofing a theorem. A problem to make a general statement regarding the numbers of all possibility of cardinality of (AUB), with A, B is set and n(A) is a and n(B) is b, is given to 60 undergraduate students who are majoring mathematics education. The statement then is analyzed and categorized into 0 to 5 based on the developed framework. The result shows that most students’ answers are categorized as 0 (29 answers), while only 6 answers that can be categorizes as 5 (well structured). As for category 2, 3, and 4 there are 3, 6, 16 answers representatively. And there is no students’ answer which is categorized as 1. The mistakes occur are: students do not fully understand the mathematics notion so the theorem made is different from the one that being asked, the given condition is not general enough (does not represent all case), the conclusion made is false. Yet, there is no conclusion that is unrelated to the condition.