NK cell leukemia is a rare malignancy which is highly aggressive and usually fatal even after treatment. We report a case of a patient who was diagnosed as NK cell leukemia with secondary hemophagocytic lymphohistiocytosis (HLH) and successfully treated with allogeneic hematopoietic stem cell transplantation (HSCT). A 16-year-old male visited hospital with persistent fever. For last 3 weeks he also had symptoms of night sweat, fatigue, easily bruise and poor oral intake, and hepatosplenomegaly was detected on physical examination. Complete blood count showed neutropenia and thrombocytopenia, and peripheral blood smear showed relative lymphocytosis with large granular lymphocytes. Ferritin level was also elevated. Bone marrow examination showed increased number of large abnormal granular lymphocytes (11.6% of total nucleated cells) and histiocytes with increased hemophagocytic activities. Immunohistochemical staining of bone marrow section showed increased CD3, CD56, EBV early Ag ribonucleic acid (EBER), and granzyme-B. Aggressive NK cell leukemia with secondary HLH was diagnosed. He underwent allogeneic HSCT from unrelated donor following chemotherapy, and now alive without disease with 1 year and 10 months follow up duration after HSCT. Due to the rarity and high mortality of this disease, there are few case reports about NK cell leukemia. Our case shows a case of NK cell leukemia treated with allogeneic HSCT.