This study describes working memory and developing and validating of an L1 Persian reading span test for the measurement of working memory of L1 Persian EFL learners. The test, which included 64 Persian sentences, was developed based on Daneman and Carpenterâs (1980) reading span test. The shortcomings of the test were identified and removed over three pilot studies on 74 participants. The final test was used in a study with 140 participants at three different proficiency levels. The results of an item analysis, as indicated by Cronbachâs Alpha, displayed an internal reliability of .844 and .790 for the RST processing and recall scores respectively. This suggests that the newly developed test is reliable enough and could be used to measure working memory capacity for future L2 studies. This study also provides a clear procedure for the development of a reading span test for speakers of other languages.