Introduction: The teacher plays a significant role in the intellectual development of the pupils, using various assessment and teaching styles to improve pupils' performance in school subjects. The study therefore investigated the effect of some teacher related factors: teacher level of awareness of assessment style, teacher assessment style, teacher personal characteristics (qualification, year of experience and teaching styles) on pupils' achievement in Social Studies in primary schools. Method: The study was a correlational type. Multi-stage sampling technique was employed in selecting three states from the existing six states that constitute Southwestern zone in Nigeria. Three instruments were used in the study. Three research questions emanated from the study and were analysed using mean, standard deviation, graphical illustration and multiple regression analysis. Results: Results revealed that Social Studies teachers engaged more in formal authority and facilitation teaching styles. There was a significant relationship among teacher related factors and pupils' achievement in Social Studies. Only two predictors had significant relative effect on pupils' achievement in Social Studies. Discussion and Conclusion: Based on these findings, teaching and assessment styles of teachers are crucial in the learning of Social Studies. Workshops and seminars should therefore be organized for teachers on various teaching styles. (Contains 4 tables and 1 figure.)