The aim of this study was to explore the level of critical thinking ability of secondary school students, and to investigate the nature of the differences in this ability which may be attributed to gender, class, major and level of achievement. In order to measure the ability of critical thinking, the researcher administered Abdul Salaam and Suliaman's test (1982) which was adapted to uit the Omani environment by Al-Hashmi (2003). The present researcher used Cronbach's alpha to measure the reliability of the test employing a pilot sample of 101 students drawn from the eleventh and twelfth classes of the general education system. The sample of study consisted of 925 students which represented 5% of the population of the study. In order to answer the first research question: "to what extent do the students of the eleventh and the twelfth classes (from the arts and science majors) acquire critical thinking skills?" Means and standard deviations were computed using the grades of the total test and each of its five subtests. The "one sample t-test" was used to compare between the means of the students in the two classes and the theoretical mean. This comparison was performed for the overall test and in each subtest separately. It was found that the differences were significant at the 0.001 level where the means of the twelve class students were higher than the theoretical means in four of the subtests while the means of the eleventh class students were higher than the theoretical means in three of these subtests. To identify the significance of the differences which could be attributed to gender, class, and major, three-way factorial analysis of variance was used. The result was that, there were significant gender differences in favor of females, and significant class differences in favor of the eleventh class. But, the results showed no statistical differences with respect to major. However, there was a significant interaction between the variables of class and major. To identify the correlation between critical thinking and academic achievement, Pearson's correlation coefficient was computed for each class and major separately. The results were that, all correlation coefficients between critical thinking and academic achievement were positive and significant in both classes (eleventh and twelfth) as well as the two majors (arts and science). The results of this study were interpreted in the light of the theoretical frame and previous research studies. Finally, the researcher provided some recommendations which were based on his results,