This study aimed to investigate the psychometric properties of motivation questionnaire available on the student's background questionnaire in TIMSS 2011 in Oman. The sample of the study consisted of 326 students from grade 8 who were involved in TIMSS 2011 in the governorate of Dhahira, Sultanate of Oman. The validity of the scale has been verified in several ways, as the construct validity which has been verified through confirmatory factor analysis. The results showed validity of the internal structure of the scale, as the items where saturated with six factors. Moreover, the convergent validity was verified of its association with the self-description questionnaire (Marsh, 1988), and the results revealed the presence of non-statistically significant relationship between the two scales. The reliability has been verified through assessing the internal consistency where the value of the motivation scale reliability was (0.782). On the other hand, the stability of the scale has been verified by using the test re-test method, the stability values ranged between (0.027) and (-0.198) indicating weak correlation coefficients. This result was explained by applying t-test between the first and the second application and the results came to confirm the presence of statistically significant differences between the two applications and in favor of the second application. The study concluded that the concept of motivation is unstable among the Omani students. The contradicting results regarding the reliability and validity of motivation scale in TIMSS 2011 can't be referred to the weakness of factorial structure of the scale but there were other explanations of the results.