Abstract: This study assessed the Effects of Competency – Based Curriculum (CBC) on Students’ Education Achievement in Secondary Schools in Arusha City Council, Tanzania. The objectives were to: assess the implementation of CBC in secondary schools and examine the extent to which the implementation of CBC affects students’ education achievement in secondary schools in Arusha City Council. Social Constructivist Theory by Theodore Burghard Hurt Brameld guided the study. Convergent mixed methods design was adopted. Datawas collect through questionnaires and interviewguide. The sample size consisted of 142 respondents; 30 teachers, 90 students selected by simple and stratified random sampling techniques. Purposive sampling technique was used to select 10 Heads of schools, 10 SQAO, one WEO and one DEO. Validity was established through expert judgment whereas reliability (QS r=0.85 and QT r=0.82) were determined using test-retest technique and Cronbach Alpha Method. Qualitative data were analyzed thematically alongside the research questions and presented in narrative forms while quantitative data were analyzed using descriptive statistics in the SPSS version 21 and results were presented in tables of frequencies and percentages.The findings show that the use of different teaching and learning methodologies such as learner centred approach, discussions, exhibitions and practical activities were encouraged. Moreover, CBC implementation is essential for the student’s future career development through self-employment, creativity, innovation, talent exploration and expansion of multiple competencies necessary for sustainable development. The study recommended that, CBC should be effectively implemented in all secondary schools for students to develop necessary skills needed for personal transformation.