The study aimed to investigate the efficacy of using the Flipped classroom strategy on English grammar achievement of the 1st Secondary grade student in Petra Jordan. To achieve the purposes of the study a quasi-experimental method was used. The sample of the study consisted of (60) male and female students in Petra schools which was intentionally selected; The participants were distributed into two groups: an experimental and a control group. The experimental group included (30) male and female students who were taught using Flipped Classroom Strategy, while the control group which included (30) male and female students was taught using the conventional method. The validity and reliability of the study instrument i.e. an achievement test for grammar were verified. The results of the study indicated that there were significant statistical differences at (α≤0.05) for applying the Flipped Classroom Strategy on students’ achievement in English grammar for the 1st secondary Grade students in favor of the flipped classroom strategy. The results also showed no significant statistical difference at (α≤0.05) in the students’ post-test in the experimental group due to gender.  Recommendations included using the flipped classroom strategy in teaching English grammar instead of the conventional method.   Keywords: Flipped classroom Strategy, English Grammar, Achievement.