The present study aimed to reveal the nature of irrational thoughts affecting academic achievement motivation among tenth grade students in Muscat Governorate, and the researcher used the descriptive relational approach. The sample of the study consisted of (392) students including (194) males and (198) females, were randomly selected from schools all over states of Muscat Governorate, and the researcher has built a scale of the irrational thoughts affecting academic achievement motivation. The new scale included (43) phrases were distributed among seven ideas for irrationality are: Academic self-contempt, innate capacity and reliability, the speed of acquiring knowledge , outstanding students hare special abilities, not the importance of education to build the future, fear of the consequences of success, modeling negative. The researcher – used a scale of academic achievement motivation Prepared by (alhamid, 1996). He measured the validity and reliability of the two instruments and showed that they have a good degree of validity and reliability. Data processing was statistically using the (spss) statistical software. The researcher used the arithmetic mean, Standard deviation, and t- test, and pearson correlation coefficient, and study found the following: 1. The most common irrational thoughts among tenth grade students in Muscat governorate is the negative modeling, followed in second place possess outstanding students to special abilities, and in the third the speed of acquisition of knowledge, and ranked fourth academic self-contempt. 2. The were statistically significant differences at the level of significance of less than (0.01) in the average tenth grade students in the level of irrational thoughts attributed to the different gender (male- female), and in favor of males. 3. The were statistically significant differences at the level of significance of less than (0.01) in the average tenth grade students in the level of academic achievement motivation due to the different gender (male- female), and in favor of females. 4. There is an inverse correlation statistically significant difference at less than (0.01) between irrational thoughts and academic achievement motivation. 5. There is an inverse correlation statistically significant difference at less than (0.01) among all ideas and academic achievement motivation, and that the first ides which is academic self-contempt is the closest idea to the academic achievement motivation, followed by the third idea which is the speed of acquiring knowledge, in the second place. Then modeling negative comes in third place.