The purpose of this study is to explore the effects of guided, inquiry-based laboratory activities using the Science Writing Heuristic (SWH) approach and self-evaluation on students' science achievement. The study involved three sixth grade classes studying an electricity unit taught by the same primary school teacher. Before the study began, one class was randomly selected to be the control group, and the other two classes were selected to be treatment groups. In the control group, students were instructed using a traditional didactic approach. Treatment groups engaged in guided, inquiry-based activities via the SWH approach. One treatment group was randomly selected to complete a self-evaluation of their SWH reports. Data collection tools included a baseline test at the beginning of the study to establish three skill-based groups and unit-based pretests, posttests, and retention tests. The Cronbach's alpha reliability of the electricity test was 0.91. Results indicated no significant mean differences among groups on pretest measures for the unit. Analysis of post and retention tests indicated that students in the SWH and self-evaluation SWH groups scored significantly higher than the students in the control group.