The effectiveness of using the PDEODE Strategy in Arabic language to develop the Grammatical Inference skills among prep stage students. Abstract: This study aimed at developing the grammatical inference skills of the second-year prep stage students, those skills necessary for them in studying the linguistic structures and grammar rules in the Arabic book. The study sample consisted of (60) second year prep students At Belbeis Administration, Sharika Governorate who were divided into two groups: an experimental group that studied the grammar topics determined in the school book by using the PDEODE Strategy, and another control one studied the same topics in the usual way without any experimental treatments. The research problem was that there were obvious weakness and deficiency among the second-year prep students in the grammatical inference skills, those skills necessary for them to understand grammar topics and study its issues, and that the skills were revealed by the pilot study conducted by the researcher and confirmed by the results of previous relevant research and studies. The research has been based on preparing a list of the grammatical inference skills required for second prep students and grammatical inference skills test to measure those skills. The results of the study showed that there was a statistically significant difference at the level of (0.01) between the mean score of the experimental and control groups in the post-administration of the grammatical inference skills test as a whole, in favor of the experimental group and there were statistically significant differences at the level of (0.01) between the mean scores of the experimental and control groups in the post-administration of the grammatical inference skills test in each sub-skill separately in favor of the experimental group, which indicates the effectiveness of the PDEODE strategy in developing the grammatical inference skills of the study sample. Key words: PDEODE Strategy ، Grammatical Inference ، Prep Stage Students . ــــــــــــــــــــــــــ