We studied the direct anti-tumor effects of ubenimex on five human lung cancer cell lines; ABC-1, RERF-LC-OK, RERF-LC-MS (adenocarcinoma) and SQ-5, EBC-1 (squamous cell carcinoma). Ubenimex dose-dependently inhibited the growth of these cancer cell lines except RERF-LC-MS. The results indicated that lung squamous cell carcinoma cell lines were more sensitive to ubenimex than lung adenocarcinoma cell lines. Coincidentally, histological observation by Hematoxylin eosine (HE) staining revealed that ubenimex induced nuclear condensation and apoptic body in the cancer cell lines. Immunohistochemical study showed ubenimex-treated cells expressed LeY antigen which is a useful phenotypic marker predictive of apoptosis. The induction of DNA fragmentation was also observed in the ubenimex treated cancer cell lines by ELISA. We conclude that ubenimex exhibits its direct anti-tumor effect against non-small-cell lung cancer cell lines, more effectively against squamous carcinoma cell lines, through the induction of apoptosis.