The purpose of the study was to assess the impact of constructivist approach on students’ achievement in biology teaching. The study carried with three research questions what are the students’ views of the reasons they face difficulties in learning some biological topics. The second question is does constructivist teaching method bring about a significant improvement in grade (10) senior (2) student biology test score? Question three what are the major challenges for teachers in teaching constructivist approach. Two government schools were selected from Juba town Jubek state south Sudan based on their location and time the schools were Juba Girls secondary school, and supiri secondary school. The study was carried out with a mainly quantitative research but used qualitative research as well. The study design was aquasi experimental design. 120 students from senior (2) (grade 10) in these two schools and five biology teachers were participated in the study. The instruments used were a questionnaire, an achievement test, lesson plan, interview questions. The quantitative data was analyzed using SPSS version 23, the interview data analyzed qualitatively. The variable treated was teaching method. The result showed seven main reasons the students gave regarding their difficulties in learning some biological topics. the constructivist approach showed an effectiveness in improving students scores in biology teaching, regarding their experience in applying constructivist approach some have an idea about it and others experience it but for a short period of time. On the challenges in applying constructivist approach, the teachers stated the main challenges as class size, competent teacher to teach and time of the lesson, but the researcher reflected on the challenges in biology teaching as introduction of the approach, lesson plan, role of the teacher as guidance and individual differences. The study recommended, introduction of a course regarding the application of approach in teachers institute and colleges of education, building more secondary schools to reduce the over crowdedness