The main purpose of this study was to assess the effects of child labour on primary education development in Tanzania. The research was carried out in Mwanza city where the study focused on three specific objectives; Identification of the causes of child labour, assessment of the effects of child labour on primary education development in Mwanza city and evaluation on the government measures imposed to combat the problem. Mixed research approach and ex- post facto design were employed whereby questionnaires and interview guide instruments for data collection were used. A sample size of 198 respondents was included involving pupils, education officers and social welfare officers. Chi - square through SPSS version 17.0 and ATLAS program were used to analyze the data. The main findings of the study were 82 (52%) of students of whom 42 (51%) boys were not affected by child labour while 40 (48%) boys were affected. Female students were 73 (48%) involved in the study; whereby 55 (75%) were not affected by child labour. Only 18 (25%) girls were affected. With these statistics it is clear that; boys were much affected by child labour compared to girls in primary schools. Furthermore the study revealed that poor economy of the family, family conflicts, family separation as well as poor willingness of the politicians are the main causative of child labour. The researcher concludes that basically child labour is caused by poverty of the individuals in the society which in turn affects pupils’ academic performance. Researcher recommends that, xvi there is a need of cooperation between the government, society, family members and other educational stakeholders to ensure this problem is addressed adequately in all corners of the society.