Language assessment knowledge (LAK) is essential for EFL teachers in developing sound assessments and making informed decisions about the progress and achievement of students. It is also essential to have sound measures of teachers' LAK to help them improve their professional accomplishments. To address the issue, this study was designed in multiple stages to develop a test to measure EFL teachers' LAK. First, EFL teachers' needs were identified using an already validated questionnaire (Farhady & Tavassoli, forthcoming). Second, the topics identified through needs analysis were crosschecked with the topics in the assessment textbooks to assure appropriate content coverage. Third, based on the data obtained from needs assessment of 246 EFL teachers and the content of popular language assessment textbooks, a scenario-based LAK test with closed-item formats was developed in six parts, each focusing on one major area of language assessment. Fourth, after having the test reviewed by experts, it was revised and piloted with 50 EFL teachers, necessary revisions were made based on the results of piloting, and the test was finalized and administered to 164 EFL teachers. The data analysis revealed that contrary to teachers' claims in needs assessment, the majority of them had low levels of LAK and they showed willingness to improve their LAK. Further, possible relationships between teachers' LAK and their gender, field of study, educational level, teaching context, and teaching experience were investigated. The findings are presented and implications and applications of the study are explained.