The main purpose of the study was to investigate the effectiveness of conceptual change oriented instruction over traditionally designed science instruction on 8th grade students' understanding of fluid force concepts. Participants of the study were fifty five 8th grade students from two classes in a private elementary school. This study is a quasi experimental pretest-posttest control group design. Both groups were administered Fluid Force Concept Achievement Test as pre and post test. Also Logical Thinking Ability Test was used at the beginning of the study to determine students' reasoning ability levels. ANCOVA was used for testing the hypotheses of the study. The ANCOVA results showed that the post test mean scores of CCI (Conceptual Change Oriented Instruction) group and TSI (Traditional Science Instruction) group with respect to achievement related to fluid force concepts were significantly different. CCI group scored significantly higher than TSI group. In addition, logical thinking ability accounted for the significant variation in achievement related to the fluid force concepts. The CCI lesson group caused a significantly better acquisition of scientific conceptions related to fluid force and elimination of misconceptions than the TSI lesson group.