The purpose o f this study is to Find out the effects o f cognitive proficiency levels o f primary school science teachers on their classroom performance in terms o f the proper use o f equipment and laboratory applications. The subjects o f the survey were 32 primary school teachers in Yozgat. In this study a descriptive method was used. Scores obtained from the Cognitive Proficiency Test and the In-Class Observation Form provided the data. A meaningful relationship between teachers' scores from the Cognitive Proficiency Test and those from the In-Class Observation Form was found; the correlation score was 0.93.