Theme-based learning (TBL) refers to learning modes which adopt the following sequence: (a) finding the theme; (b) finding a focus of interest based on the theme; (c) finding materials based on the focus of interest; (d) integrating the materials to establish shared knowledge; (e) publishing and sharing the integrated knowledge. We have created an on-line system which supports the TBL mode to provide elementary school students with Web-based thematic learning. This study investigated the learning process of 40 fifth grade students in the natural science unit, "Stars and Sun", through the construction of conceptual maps supplemented with structured interviews with both instructors and students on the process and results of the Web-based thematic learning. The following conclusions were reached: (a) the Web-based thematic learning. system has positive effect on learners' concept learning; (b) the Web-based thematic learning first providing learners with a framework from which develops the related concepts, is a more stable learning mode; (c) the Web-based thematic learning system is suitable for students with different abilities. The middle achievement group of students is particularly suited to using a sharing and observation Web-based learning strategy. (Contains 6 tables and 11 figures.)