The research aims at investigating the effectiveness of using flipped learning strategy in the academic achievements by eighth grade basic students in the subject of science and developing their reflective thinking. To reach the objectives of the study, two null hypotheses have been constructed by the researcher. The population of the study was limited to the eighth grade basic students for the academic year (2019-2020) at (Trooska Basic School for Girls) which has been chosen intentionally. The researcher relied on the experimental design, with control and experimental groups by adopting pre-test and post-test to the variable of reflective thinking, and post-test to the variable of Academic Achievement. The experimental group has been taught according to the Flipped Learning Strategy, while the control group according to the Traditional Teaching Method. The sample consists of (41) students, distributed between two groups; experimental group (21) students, and control group (20) students. The equivalence between the two groups was conducted in a number of variables. The researcher has prepared two tools, the first one an academic achievement test to measure the academic achievement, which consists of (30) multiple choice question items extracting difficulty coefficient, discriminatory strength and effectiveness of wrong alternatives, second tool is reflective thinking scale which consists of (35) items distributed to five dimensions (Visual vision, Paralogisms revealing, Conclusion, Providing convincing explanation, Proposed solutions). The validity and reliability of the tools have been duly confirmed. After completing the application of the tools, the data has been obtained by using statistical means, by using (T-test). Results have shown that using Flipped Learning Strategy in teaching the science subject has positive impact on increasing students' academic achievement and developing their reflective thinking, but using the traditional teaching method did not show any developing in their reflective thinking. In the light of research results the researcher has recommended the importance of using the flipped learning strategy which is considered a modern strategy, also procedures for some further future studies have been suggested.