Abstract
The purpose of this study is to improve critical thinking of students' skills in Complex Analysis lectures through discovery learning combined with numbered head together type Cooperative Learning. This research uses a class action research approach. Each cycle consists of several stages, namely: planning, action, observation, and reflection. To obtain research data two types of research instruments were used. The research instrument in question is the observation sheet of the implementation of learning and observation guidelines as well as test questions on critical thinking skills at the end of the cycle. The results of this study indicate that The average score of critical thinking of students skills was 96.40 in the first cycle increased to 126.40 in the second cycle, but the average percentage of critical thinking of students skills increased from cycle I to cycle II, namely 62.25% in cycle I increased to 81.70% in cycle II.