Diaphragmatic hernias, whether congenital or acquired (traumatic), are rarely observed in the horse. Acquired diaphragmatic hernias typically occur secondary to trauma or an increase in intra-abdominal pressure due to falling, heavy exercise, or parturition. Diaphragmatic herniorrhaphy is difficult to perform in adult horses and the horses with symptomatic diaphragmatic hernias usually die. A 10-year old, 340 kg, Jeju horse (crossbred) broodmare with sudden onset of gait disorder and a moderate emaciation was examined. Findings on physical examination included conjunctivitis, dehydration, shallow breathing, dyspnea, weaken heart beat, lack of auscultatable sounds from the gastrointestinal tract, and anorexia. Rectal temperature was 38.4℃ and respiratory rates were moderately increased. There were slight signs of acute colic. The broodmare died one day after non-specific treatment of fluids, nutriment, antibiotics and non-steroidal anti-inflammatory drug. The cause of death was strangulation of the small intestine through a diaphragmatic hernia. The rent was about 2 cm in diameter and located in the central right part of diaphragm. Around 60 cm of small intestine was protruded into thoracic cavity through the rent. The cause of the hernia could not be ascertained. The broodmare had been pastured with many other horses, and the groom had not noticed any aggressive behavior among them. It was, however, speculated that trauma by stallion's attack may have been the cause of the diaphragmatic hernia, because the new horse may be the object of behaviors ranging from mild threats to seriously aggressive kicking, squealing, rearing, and biting.