The present study aimed at constructing a formative assessment inventory for assessing EFL instructors' conception and literacy about the issue under study. Therefore, 302 male and female Iranian EFL teachers took part in the study in a random way to fill out the researcher made questionnaire. The first draft of the scale consisted of two main categories, tacit (theoretical) and practical knowledge along with 26 items. After employing EFA and CFA, it was revealed that the questionnaire consists of a high validity. Also, for having a better fit for model and construct validity, some of the items were omitted out of the first draft of the questionnaire because of having low loadings. In addition, the reliability of the questionnaire was gained by running Cronbach's Alpha which was 0.855. At the end, the results for EFA revealed that the two factors can be regarded as the two constructs that the test claims to measure, namely: (1) Tacit knowledge, and (2) practical knowledge. Among the 17 items of the "Tacit knowledge category" five items (9, 14, 15, 16, and 17) were excluded because of loadings less than 0.30.