This study aimed at knowledge the effect of using guided discovery method in teaching Fekeh subject of Islamic Education on the achievement & critical thinking of the 10th grade in the sultanate of Oman. This is done by answering the following Questions: 1. What is the effect of teaching Islamic Education through guided discovery method on the achievement of the 10th grade of Fekeh subject? 2. What is the effect of teaching Islamic Education through guided discovery method on the performance of the 10th grade in critical thinking scale? 3. Is there any relation between achievement & ability on critical thinking of the experimental group students whom are studied through the guided discovery method? The population of the study is 3470, of all male students in 10th grade in AlDakhyliah region in the academic year 2004/2005. The study sample was chosen from Sheikh Mohammed Bin Masoud Al-Busaidi School. Two class (each contains 33 students) were randomly selected from five different branches existed in the school. One of them was randomly chosen to be an experimental group and the other one as a control group. The experimental group studied through the guided discovery method, whereas the control group studied through the traditional method. To aimed the study, the researcher used two tools: - The first tool was the achievement test on Fekeh subject consisted of 30 questions by the researcher himself and was validated by a panel of judgments. Its reliability was obtained by using person correlation coefficient which reached 0.75 and was applied before and after the experiment. - The second tool was the critical thinking scale which was prepared by Faroq Albdulsalam and Mamdooh Suliman (1981) and was adapted to fit Oman's environment and Educational Secondary stage by Mohammed Bin Abdullah AlHashmi (2003). The researcher examined its reliability by using person correlation coefficient which was 0.79 and he applied it before and after the experiment. The main finding of the study are: • There was statistical significance differences at (0.05) between the two groups (experimental and controlled) in favour of the experimental group in the achievement test. • There was statistical significance at (0.05) between the two groups (experimental and controlled) in favour of the experimental group in the test of the critical thinking. • Then was a positive relationship between achievement and critical thinking of the experimental group. In the light of the results of this study, the researcher suggested a number of recommendations. He also suggested a conducting research in guided discovery and improvement of the critical thinking.