This study aimed to investigate the effect of teaching geometry by using guided discovery method on achievement and critical thinking, showing the relationship between achievement and critical thinking for students who are taught geometry by guided discovery method. The sample of study consisted of (70) students in grade 10 in sohar secondary school for general education. The sample was divided into two groups : An experimental group of (35) students and controlled group of (35) students. We made sure that there is balance between the two groups concerning the previous achievement and the previous critical thinking. To analyze the data the researcher used the following means , standard deviation, T-test and pearson correlation coefficient This study reached the following conclusions: - There were statistically significant differences (a=0,05) between the students means of scores in the achievement test due to the method of teaching favoring the students taught geometry by guided discovery method . - There were statistically significant differences (a=0,05) between the students means of scores in the critical thinking test due to the method of teaching favoring the students taught geometry by guided discovery method . - There is a relation that is positive and clear statistically at the level of (a=0,01) between marks of experimental group students in achievement test and critical thinking test. In the light of these conclusions, the study recommended using guided discovery method for teaching geometry because it has a positive effect on increasing achievement and critical thinking for the students.