This study was aimed at investigating the effectiveness of teaching science in Tanzanian community secondary schools: in Mwanza city. The study adopted a conceptual framework based on effectiveness of teaching science adopted from Lockheed and Verspoor. The study employed two research designs namely; the cross sectional design and the quasi – experimental design. Eight government secondary schools were researched, five schools in treatment group and three schools in control group. The sample size had 384 respondents comprising of science students, science teachers, head of schools and the District education officers. Data were collected through questionnaire, interview, observation and documentary review techniques. The main findings of the study were that, most of secondary schools in treatment group had no laboratories. The findings indicated that, participatory and non participatory teaching strategies were used in teaching science in treatment and control groups. It was further revealed that performance for students in science subjects was good and satisfactory in treatment and control groups. It was shown that there was a shortage of science teachers in treatment and control groups. The study recommended the following to be done by education stakeholders in treatment and controls groups; establishing laboratories for science subjects, participatory methods should be encouraged in teaching science and recruiting more qualified and competent science teachers to raise the performance in science subjects.