Assessment of grammatical knowledge is a rather neglected area of research in the field with many open questions (Purpura, 2004). An overview of the construct of grammar over time indicates that the dimensions of grammatical knowledge have not been adequately addressed in most of the models of grammar. The present research incorporates recent proposals about the nature of grammatical development to create a framework consisting of dimensions of complexity, range, and strength, and investigates the dimension(s) which can best predict the state of grammatical knowledge of EFL students. To this end, the specifications of a test of grammatical knowledge were drafted and reviewed by (N = 7) of trained specifications reviewers. The specifications were revised and the test was administered to 158 English learners with mixed proficiency levels. The items were analyzed using classical test theory and Rasch model. The results of stepwise regression indicate that the model that best predicts grammatical knowledge of lower ability leaners includes range and strength, while for the higher level strength and complexity are the best predictors. This finding implies that specifications of a grammatical knowledge test may include use of a combination of item formats in order to measure different dimensions.