Deep vein thrombosis (DVT) is the formation of blood clots (thrombus) in the deep veins. Anticoagulation therapy is the primary treatment for DVT. A 70-year-old woman was admitted to our Emergency Department due to a 3-day history of pain and swelling in the right leg. She was initially diagnosed to have DVT, but severe leg swelling with acute compartment syndrome developed after anticoagulation therapy. Her activated partial thromboplastin time (aPTT) was more than 120 seconds. A computer tomography scan of her lower limb showed the cause to be hematoma, which was then treated with fasciotomy. To avoid heparin related complications such as hematoma, aPTT should be carefully monitored and heparin dosage should be adjusted for risk factors.