In recent years, it has been reported that thrombus formation plays an important role in the development of stress ulcer. In our study, the effect of fraxiparine, which is a low molecular weight heparin (LMWH), on stress ulcer, induced by cold-immobilization stress in rats, was investigated histologically. Before stress, five different doses of fraxiparine (500, 250, 100, 80, 50 U/kg) were injected subcutaneously. One and a half hours after the injections, rats were housed in wire cages for five hours at 7-9 °C temperature. After the stress period, rats were decapitated, their stomachs were taken out and opened along the greater curvature. The mucosa was cleaned of clots and material was obtained from necrosed mucosal areas for light microscopical investigation. Bleeding and erosions in the mucosal sections were scored for evaluation. There was no difference histologically between the 500 and 250 U/kg fraxiparine applied groups and the control group. One hundred U/kg fraxiparine decreased the bleeding, but erosions were similar to those of the control group. The mucosal erosion areas were minimum, and there was no bleeding in the 80 and 50 U/kg fraxiparine applied groups. In conclusion low dose fraxiparine, that is 80 and 50 U/kg fraxiparine, is effective in the prophylaxis of stress ulcer, according to the histopathological evaluation results.