Developing critical thinking is an important factor in education. In this study, the author defines critical thinking as the set of skills and dispositions which enable one to solve problems logically and to attempt to reflect autonomously by means of metacognitive regulation of one's own problem-solving processes. To identify the validity and reliability of students' self-assessment, the critical thinking rubric and Toulmin model combination was introduced. At the end of the lesson all Toulmin model work sheets written by students were revised correctly. It was discovered that students solve problems logically. This shows that a high level of critical thinking among students is reliable and valid. [For the complete proceedings, see ED579395.]