The instrument that assesses teachers' competence on AR methodology is limited. Thus, it is one of the issues concerning evaluating the effectiveness of a professional development program on designing AR projects. It is difficult to determine how much and what teachers have learned in a course or training. Thus, this cross-sectional study aimed to evaluate further the validity and reliability of the Teacher's Competence in Action Research Questionnaire, a seven-factor instrument previously proposed by Cortes, Pineda, and Geverola (2020). This self-report scale was not subjected to confirmatory factor analysis, had less sample size, and had homogenous participants. In the present study, 450 participants, both pre- and in-service teachers and from different teaching specializations, answered the survey. The data were analyzed using the confirmatory factor analysis method through the Maximum Likelihood approach. Four model fit indices recorded satisfactory results (CFI = 0.890; TLI = 0.884; RMSEA = 0.072; SRMR = 0.039), thus, supporting the seven-factor scale. The standardized factor loading, composite reliability, average variance extracted, and Cronbach's alpha coefficient/s of the entire scale and within subscales also provide evidence of the convergent validity and reliability of the scale. There may be an issue in the discriminant validity of the scale, but the conceptual distinctions of each factor as supported by theoretical foundation and arguments provide a principal reason for retaining all the items and factors.