This study investigated the relative effectiveness of the constructivist-based instructional model on senior secondary students' achievement in Biology in rivers State, Nigeria. The study study adopted a quasi experimental design. In each of the sampled schools, one intact class was randomly assigned the experimental group and the other, the control group. The experimental group was taught using constructivist instructional model while the control group was taught using traditional lecture method. Post-test was given to both groups after the teaching. The results were analysed quantitatively for overall performance using mean, standard deviation and t-test. The result of this study revealed that subjects taught using the constructivist-based instructional model performed significantly better than those taught the same lesson using traditional lecture method. Hence, constructivist instructional strategy is feasible for inclusion in Biology curriculum. The researcher recommended that the constructivist teaching strategies be adopted at the secondary level of education in the teaching of science subjects as this may enhance quick and effective mastery of difficult concepts.