This study investigated the frequent grammatical errors, found in the writings of Arab students' taking English writing courses in AL-Hussein Bin Talal University Learners' errors were considered positively as the best sources to identify students' limitations in English writing. Therefore the present study intended to investigate the grammatical errors of Arab students' writings in English in AL-Hussein Bin Talal University and to see if there are any differences in the grammatical errors according to year of study. To conduct this study data was collected from the writing sessions of writing classes that were taught during the fall semester of 2019. The data was collected, analyzed and categorized from students, all majoring in English Language and Literature and ranging from freshman to seniors. A Grammar test Questionnaire designed by the researchers was distributed to the students in these writing sessions. The results showed that the most frequent grammatical error was with the verb tense on a mean of (3.75), followed by errors in the article on a mean of (3.62), wrong word order on a mean of (3.57), noun ending on a mean of (3.40) and least was sentence structure on a mean of (3.39). The results also showed that the seniors on the grammar test on all its parts did better than the freshmen, juniors and sophomores that are the least problems were found among the seniors.