This study investigated the effects of chemical concept understanding level on students’ achievement in biochemical topics (photosynthesis, respiration, diffusion and osmosis, carbohydrates, protein, fats and enzymes) in some selected secondary schools in Delta State of Nigeria. The study made use of mixed method research approach. Quantitatively, the design of the study was quasi experimental non-randomized pre-test and post-test control group design. A case study Embedded Design was adopted to take care of the qualitative aspect of the mixed method study. Kuder-Richardson formula 20 (K-R20) was used to calculate the reliability coefficient of the test. The internal consistency of reliability co-efficient was calculated to be 0.76 that made the instrument to be accepted as reliable for the study. The population of this study consisted of all senior secondary (II) biology students in the twenty (25) local government areas of Delta State of Nigeria. Six secondary schools randomly selected from the three senatorial districts of the State were used for the study, with three (3) schools for experiments specially taught with emphasis on both ideas (concepts) and skills, problem-based interactive learning ensuring concept connections and the other three (3) for control groups. Intact classes were used for both groups. The intact sample was made up of five hundred and ninety two (592) students for the quantitative study; while two teachers and four students were purposively sampled for the qualitative aspect of the study. Four (4) research questions were raised and answered and four (4) null hypotheses were formulated and tested at 0.05 level of significance. The research instruments used were the Test of Students Understanding of Chemical Concepts (TOSUCC), Biology Achievement Test (BAT) which measured achievement in biochemical topics and Interview Schedules for both teachers and students. The quantitative data collected were analysed using Analysis of Covariance (ANCOVA) for the hypotheses and mean ratings for the research questions; and the qualitative data were analysed by coding and themes generation and interpretations. Findings show that the experimental group performed better than the control group in the biochemical topics. An effect size of Cohen`s d equal to 0.996 was determined from the mean scores and standard deviations of the experimental and control samples for the study. Therefore, the experimental group had better achievement because determined effect size seemed large enough in favour of the treatment group. No significant interaction effects of gender and location on students` level of achievement in both experimental and control groups were established by the study. The teachers in particular should use the outcome of this study to improve on their teaching, emphasizing on concept understanding in their teaching and prior learning/knowledge or pre – requisite concepts. This is because certain chemical concepts are required before the teaching of the main topics (Biochemical topics in this case). In other words, this study recommends that secondary school biology and chemistry teachers should teach for concept understanding, topics that are related to the new topics before teaching the topics. / Science Education / Ph. D. (Science Education)