Problem-solving and effective communication skills become prominent in today's educational policies and shape educational practices. This study aims to examine primary school teachers' self-efficacy perceptions regarding in-class effective communication skills and teaching 21stcentury skills. The study is correlational research in the quantitative research method. 132 primary school teachers formed the study group. The data were collected through three different tools (Demographic Information Form, In-Class Effective Communication Skills Perception Scale-ICECSPS, and Teaching 21st Century Skills Scale), the Spearman-Brown correlation analysis was used in the analysis. The findings showed that as the number of classrooms in the school increases, the correlation between teachers' ICECSPS scores and teaching 21st century skills scores was not significant; on the other hand, classroom size was an important predictive in teachers' 21st century skills and teamwork.