Cognitive Load Theory (CLT) is very helpful in designing research in mathematics education. In addition to Guest Lecturer Dr. Ouhao Chen, Ph.D. from Educational Research Department, NIE, NTU, Singapore, this CLT field of study is also pursued by a lecturer in the Master Program Pend. Mathematics UNY, Endah Retnowati, Ph.D. This guest lecture activity was held in one of the new buildings owned by the Postgraduate Program on November 23, 2018. The atmosphere of guest lecture activities becomes very lively because the speaker always involves students. The students dared to discuss and express their opinions.