The critical thinking ability is still difficult to apply in mathematics learning, especially in elementary school students. This problem is influenced by giving the routine questions for the students. So, the students are not trained in the thinking ability. This study aims to determine the improvement of students mathematical critical thinking ability after obtaining learning with Think Talk Write (TTW) model and to know the different mathematical critical thinking ability of students who get Think Talk Write (TTW) model learning and students who get conventional learning. This model emphasizes independent thinking, discussion, and writes the conclusions of thinking and talk activities. The critical thinking ability is to think deeply and need proof. The research method used is quasi experiments nonequivalent control goup design. Population in this research is V grade elementary school students in Cileunyi Regency Bandung, with sample of class V students of SDN Yasahidi and students of class V SDN Sukahati 01. The research instrument is in the form of description of critical thinking ability test given to students during pretest and posttest. The result of the research shows that (1) students mathematical critical thinking ability which get learning with Think Talk Write (TTW) model has increased it had showed by gain indeks of 0,68 ; (2) the mathematical critical thinking ability students who learned by model Think Talk Write (TTW) are better than them who learned from conventional method, it has showed by t-test of 0,024 at the posttest. So, the learning model of Think Talk Write (TTW) can enhance students mathematical critical thinking ability.