Despite the numerous endeavours made to develop questionnaires to assess learners' strategic behaviour in general and learning/use strategies across different language areas and skills, one can surprisingly find no inventories to address learners' grammar learning /use strategies. This study aims to validate a measure of additional language learners' grammar strategies. The validation of the instrument, grammar learning/use strategy questionnaire, involved three phases. To generate an item pool as the first phase, the researcher drew on Oxford's instructional modes of L2 learning, Oxford's strategic self-regulated model of learning, Larsen-Freeman's three-dimension model of grammar, grammar strategies in the inventories of language skills and Brigg's classroom task-based grammar learning strategies. The second phase of the study involved submitting the instrument to extensive piloting in an adequate sample, followed by the third phase, which comprised evaluating the psychometric properties of the revised instrument through exploratory and confirmatory factor analysis. The results indicate that the developed instrument has satisfactory psychometric characteristics and that the hypothesized theoretical model has a great fit with the data. The article closes with some tentative pedagogical recommendations and implications as well as guidelines on how grammar learning strategies could be utilized and investigated.