The purpose of the study is to compare problem based learning (PBL) and lecture-based learning (LBL) in Hong Kong secondary students' science achievement. Secondary One students were divided into two groups: group A (n=37), was taught two topics: "Human Reproduction" and "Density" through PBL; group B (n=38) was taught the same topics by LBL. Multiple choice questions and short structured response items were used to assess students' academic performance. Pre and post tests were categorized into three domains: knowledge, comprehension and application according to Bloom's Taxonomy (Bloom 1956). The results of this study suggest first that PBL is at least as effective as LBL in gaining the knowledge required to achieve the syllabus' learning objectives; secondly, the PBL group shows a significant improvement in students' comprehension and application of knowledge over an extended time. Seemingly, PBL is favored for knowledge retention compared to a more conventional teaching approach, by these early adolescent children in Hong Kong. An ongoing longitudinal study on students' interactions will further determine whether students taught through PBL develop improved learning in relation to high order skills, in a local situation which still tends to focus on factual recall but where higher skills are being demanded by systemic reform.