KML001 (NaAsO2, sodium metaarsenite, KOMINOX), a kind of arsenic compound, that has shown promising efficacy in non-Hodgkin's lymphoma (NHL) both in vitro and in vivo. In our study, the antileukemic effect of KML001 on acute lymphoid leukemia (ALL) and its mechanism of action were investigated. The results showed that KML001 inhibited cell proliferation in two types of ALL cell lines, CCRF-CEM and Molt-4. Exposure of ALL cells to KML001 induced apoptosis in a time-dependent manner. KML001 caused cell cycle arrest at G2/M phase instead of G0/G1 phase shown in other leukemia cells. In addition, we also tested the possibility of synergy of KML001 with doxercalciferol, a vitamin D2 derivative. Also, we found that a combination of KML001 with doxercalciferol showed a synergistic effect on ALL cell lines and this could be due to its different mechanism of action. Overall, our findings demonstrated KML001 could be a promising antileukemic agent especially when it is combined with doxercalciferol in ALL treatment.