The aim of the present study was to determination the prevalence of gastric ulcer in Caspian miniature horse. The design Field- laboratorial study on 23 Caspian miniature horses. Feed withdraw 12-16 hours before study and the owners or riders completed a form about housing, medication, age, sex, cell boold counts and the biochemical profile tests were carried out. Endoscope performed through the nostrils into the stomach for a systematic examination. Findings of endoscopy were analyzed by chi-squire test. Moreover, mean valus of the laboratorial values between horses with or without gastric ulcer were compared by T- test. Gastric ulcer was evident in 47. 82% of cases. However, 81. 9% and 18.1% of ulcers were in nonglandular and glandular regions of the stomach respectively. There was significant differences between glandular and nonglandular ulcers (p<0.01) and between genders (The females had higher incidence of gastric ulcer than males)(p<0. 05). Low incidence of gastric lesions were seen in horses which received antihelmentic drugs. Moreover, horses with history of long-term treatment with NSAID, had significantly higher incidence of gastric ulcer in the glandular region (p<0.05). The occurrence of gastric ulcer was associated with over exercise (p<0.05). There was no significant differences between serum Ca, Na, K, P, Mg and total protein levels in horses with or without ulcer. However horses suffered from gastric ulcer, showed a high serum levels of fibrinogen(p<0.05).