This mixed-methods study explores the factors contributing to the language teachers' effectiveness in the context of English as a foreign language. Through a systematic review of the literature, six main factors were extracted: assessment literacy, content and pedagogical content knowledge, experience, oral proficiency, personality type, and self-efficacy. In the first phase of the study, 13 experts in the field shared their attitudes towards these factors through a semi-structured interview. The data obtained from the interviews was analysed thematically to develop a questionnaire. Ninety-three language teachers participated in a pilot study to validate the newly developed questionnaire. The results were factor analysed. After the required modifications based on the factor analysis were introduced, a questionnaire with 19 items entitled "EFL Language Teachers' Effectiveness" was developed.