The aim of the current study was to verify the internal and external validity of the construct of implicit beliefs concept in the sports context, the Conceptions of the Nature of Athletic Ability Questionnaire-2 (CNAAQ-2). The data of this study were collected from 322 adolescent football players by using CNAAQ-2 and MT (Mental Toughness) Questionnaires. The CFA was employed to examine internal validity (dimensionality), and the ANOVA with the K-means cluster analysis was conducted to examine external validity (discriminability). The results of the CFA supported convergent and discriminant validity of the specific factors (incremental / entity belief) but did not support the significance of the general factor (implicit beliefs). The results of clustering showed that implicit beliefs propensity is divided into three clusters. ANOVA, which set these clusters as an independent variable, showed significant differences for the MT. Based on these results, considerations in the utilization of CNAAQ-2 were discussed.