This study aimed to investigate the effect of teaching science using activities based on CORT program in critical thinking and academic achievement among grade ten students in Oman. The sample consisted of (105) students enrolled at Shams Al-uloom Male Basic Education School (5-10) in (Sharqiah Shamal) Eastern North Region (the experimental group=51, the control group=54) in the academic year 2007-2008. For the purpose of study, the researcher constructed a guide for teacher ,who taught the experimental group. This guide consisted of theoretical background about CORT program, and plans for (37) lessons of chapters six and seven from unit three of sciences textbook. Validity of the guide was checked out through a panel of judges who have academic and educational expertise. And then the guide was modificated according to their opinions. In addition for the purpose of study two instruments have been used: First: A Watson – Glaser Critical Thinking Test which was reconstructed by Abdulsalam and Suliman (1982) to be used in Saudi's institutions. The researcher adapted it to fit Omanie's institutions, with reliability of (0.78). Second: Academic achievement test in the science, which was constructed by the researcher. The reliability was calculated using Cronbach-Alfa, and found to be (0.83). Results of this study revealed there are no significant differences at (a<0.05) between experimental and controlled groups in both Academic achievement test and post-critical thinking and its five components. However, these results indicated that there are significant differences at (a<0.05) in performance of experimental group in pre and post-critical thinking tests in evaluation of discussions competence and the total degree in favor of post-test. Also there are significant differences at (P<0.05) in performance of controlled group in pre and post-critical thinking tests iņ interpretation competence in favor of post-test. Eventually, based on results the researcher provided some recommendations like using of CoRT method in discussions in and out of groups. The researcher also recommended further studies tackling such approach.