Critical thinking has a very important role in constructing the improvement of students' ability to face the 21st century, especially in generalizing the pattern of the two dimensional arithmetic series. Since critical thinking is necessary when we try to understand and process information, put forward ideas or ideas objectively, and develop deeper insights. The purpose of this study is to analyse students' critical thinking skills based on P21. This study is a combination method in which this method is a combination of quantitative and qualitative methods. Research subjects are high school students. This subject is expected to provide an overview of critical thinking based on P21. Data collection techniques in this study are: (1) test, this technique is used to measure the ability of students in mastering the arithmetic array of two dimensions; (2) interview; it is based on the students work in solving the problem of two dimensional arithmetic series. The data result showed that in the experimental class had increased 40.85% on the indicator of effective reasoning, 37,44% on indicator of thinking system, 47,53% on decision indicator, and 42,55% on problem solving indicator. While the control class experienced a 19.5% increase in the effective reasoning indicator, 0.07% on the indicator of the thinking system, 0.02% on the decision making indicator, and 0.02% on the problem solving indicator.