The use of different teaching methods has resulted in different quality and quantity of students' knowledge. For this reason, it is important to constantly review the teaching methods and applied most effectively. One way of determining instruction efficiency is by using cognitive load and student achievement. Cognitive load can be generally defined as a requirement for working memory resources that are needed to meet the goals of the cognitive activities in certain situations. So, the aim of this study is examining the instructional effectiveness of two teaching methods that are commonly used in teaching physics in the Republic of Serbia, using experiments and traditional method. We get that students which used experiments in teaching physics have achieved higher performance and less cognitive load than students which used traditional methods. So, we get the greater effectively of the teaching method which contains experiments.