This study aimed to investigate the psychometric properties of Marsh's Self-Description Questionnaire (SDQ-1), and to test some hypotheses that emerged from the self-concept theory, including the multidimensionality, hierarchy of self-concept and the hypotheses of the internal/external frame. The sample of the study consisted of 1061 male and female students from grades 7, 8 and 9 in the Governorate of South Batinah, Sultanate of Oman. The validity of the scale has been verified in several ways, as the face validity and the construct validity witch has been verified through confirmatory factor analysis. The results showed a hierarchical construct of self-concept as the items saturated with seven initial factors that produced two secondary factors: academic self-concept and non-academic self-concept. The results also provided support to the hypotheses of the internal/external frame of reference model. Furthermore, the convergent validity of the scale has been verified as SDQ-1 correlated strongly and significantly with a self-esteem scale. The divergent validity of the scale has also been verified by correlating SDQ1 with an anxiety scale- that has shown a weak negative correlation with self-concept. The reliability has been verified through assessing the internal consistency. The internal reliability values (alpha) of the seven sub-scales of the SDQ-1 ranged between (0.74) and (0.92) and with a 0.95 for the overall. On the other hand, the stability of the scale has been verified by using the test re-test method. The stability values of the seven sub-scales ranged between (0.74) and (0.91) and it was 0.87 for the overall SDQ-1. These high correlation coefficients indicated that the scale is stable. The results were discussed in light of the theoretical framework and previous studies and recommendations were proposed for the field of .education.