This study determines the effect of constructivist-based teaching approach on the cognitive and affective levels of conceptual understanding of Marine Biology of BS Biology students. The study made use of the pre-test-posttest one-shot experimental design. The participants were selected using purposive sampling. Data and other information needed in this study were gathered mainly through conceptual understanding test, formal interviews and journals. And to treat these data, the mean scoring system and t-test for dependent samples were used. Results revealed that the extent of cognitive level of conceptual understanding in Marine Biology of the students based on their journals was observed to a very great extent. On the other hand, the extent of affective level of conceptual understanding of the students in Marine Biology as revealed in their journals was observed to a great extent. Moreover, the findings also showed that there is significant difference on the students’ level of conceptual understanding based on their mean scores in the test after using the constructivist-based teaching approach. Lastly, it is also found out that the students learned and appreciated Marine Biology through constructivist-based teaching approach based on the interviews conducted to them.