Extranodal NK/T cell lymphoma (ENKL) is relatively rare. The prognosis of ENKL has been improved by the development of combined therapies such as RT-DeVIC; however, relapsed and resistant cases to initial treatment have still poor outcomes. We report a rare case of ENKL with leukemic conversion who relapsed 5 years after autologous transplantation. A 55-year-old woman was diagnosed as ENKL in 2005, and treated with a total of 60 Gy radiotherapy and three cycles of DeVIC (DEX, VP-16, IFM, CBDCA). She reached complete remission and received successful autologous transplantation with MCEC (MCNU, CBDCA, ETP, CPM). Five years later, she was referred to emergency room and admitted because of intractable nasal bleeding in 2010. Her complete blood counts were as follows: Hb 5.1 g/dl, Plt 3.0 × 104/μl, WBC 81,100/μl (abnormal cell 93%). The phenotypes of abnormal cells showed the same as those of the initial lymphoma cells, which included CD2 + , CD3 − , CD4 − , CD8 − , CD16 − , CD56− and granzyme B + , and their karyotype showed additional complicated abnormalities. T-cell receptor rearrangement and EBER-ISH were negative. She showed no lymphadenopathy or local recurrences. She underwent DeVIC therapy, which resulted in partial effects and rapid tumor growth. Then, she was treated with SMILE (DEX, MTX, IFM, L-asp, ETP) regimen. Thereafter, second complete remission was achieved and additional complicated chromosome abnormalities were not detected. This case suggests that SMILE therapy is one of the hopeful regimens for relapsed ENKL with leukemic conversion. Further studies are needed in developing useful and epochal regimens for relapsed ENKL.