The aim of this study is to investigate the effect of web-based instruction with web-based instructional material, which covers"Matter and Heat" unit on 6th grade Science and Technology Course Workbook, on students' academic achievement. Nonequivalent control group, which is a model of Quasi-experimental method, was used in the study. The research was conducted with 102 sixth grade students attending two different primary schools in Artvin in spring semester of the 2010-2011 school years. As data collection tools, "Matter and Heat" unit achievement test was administered in two schools as pre-tests and post-tests and open-ended questions were used after implementation. Between the experimental and the control group students' post- test scores, significant differences were found in favor of the experimental group in which web-based instruction was used.