Background: Aphthous ulcer or recurrent aphthous stomatitis is one of the most common mucosal disorders of the mouth. The exact etiology of aphthous ulcer is uncertain, but precipitated factors include stress, trauma, food sensitivity, and genetic predisposition. Stress and anxiety have a role in the onset and recurrence of aphthous ulcers. Aim of study: This study aims at clarifying the association between stress and aphthous ulcer and the influence of other triggering factors. Study design: observational cross-sectional study which was conducted on medical students of Medical College of Tikrit University (n/566), Patient age range from (19-24) years. The data were collected using questionnaires involved perceived stress scale (PSS) by Cohen. Results: In this study the frequency (31%) (176/566) of students has aphthous ulcer, (62%) (110/176) of them were female, and (37%) (66/176) of them were male. Perceived stress scale was high among the medical students, in the male (10%) (7/66) were under mild stress and (56%) (37/66) were under moderate stress and (33%) (22/66) were under severe stress. While in female (6%) (7/110) were under mild stress and (79%) (87/110) were under moderate stress and (14%) (16/110) were under severe stress. Conclusion: The prevalence of aphthous ulcer was high among medical students. With a highly significant association with stress.