The purpose of this of this study was to investigate the influence of in service training on public primary school teacher’s continuous professional development in Tanzania with specific reference to Magu district. Four specific objectives guided the study namely: To find out how in service training is delivered to primary school teachers, to identify the types of in service training provided to public primary school teachers, to examine the frequency by which public primary school teachers have attended in service training in Magu district, and to measure the effect of in service training upon primary school teachers’ cognitive and affective domains. Quantitative research approach was employed in the study; the sample involved 115 public primary school teachers. Data were collected through open-ended and closed questionnaire. Simple random sampling was used to select respondents of the study. Pie charts and tables were used to analyze quantitative data. The find revealed that in service training was used to cover different types of training like Diploma. Advanced Diploma and Degree, Also in service training upgrading qualified teachers, enables teachers to upgrade to higher level of qualification usually associated with pay. Furthermore it discovered that most teachers attended in service training for continuous professional development usually inform of short courses, seminars are no linked to specific qualification. Furthermore in service training for teachers improved teacher’s pedagogical skills and improve teacher’s adaptive skills. Thus, it is recommended that head of schools has to establish teacher’s professional development policy which will guide in service training to teachers since the study was conducted in one district involving only six public primary schools, there is a need to conduct a similar study using large sample for generalization purpose.