Despite a general consensus on the educational effectiveness of inquiry-based instruction, the enacted type of inquiry in science classrooms remains debatable in many countries including Saudi Arabia. This study compared guided-inquiry based teachers' professional development to teacher-directed approach in supporting Saudi students to understand the topic of density. One hundred and seven, sixth-grade, Saudi students in six classes were randomly assigned, by school, to one of two conditions (guided or teacher-directed condition) while they studied the same unit on density in their science curriculum. The three teachers in the guided condition attended an intervention on using guided-inquiry activities to teaching a unit on density. The three teachers in the teacher-directed condition used their regular approach to teaching the same unit. Pre- and post-tests of the students' understanding and explanation of density was adopted for the study. A one-way analysis of variance (ANOVA) and repeated analyses were performed to assess the students' understanding and explanation of density. In comparison to the teacher-directed condition, the students in the guided-inquiry condition demonstrated significant improvements in both conceptual understanding and their levels of explaining the concept of density.