Drug insensitivity or resistance is a major obstacle for successful treatment of acute myeloid leukemia. MicroRNAs (miRNAs) are small non-coding RNA molecules. Increasing evidence suggests that miRNAs modulate cellular sensitivity to anticancer drugs. We used a specific anti-miR-21 oligonucleotide (AMO-miR-21) to sensitize leukemic HL60 cells to arabinosylcytosine (Ara-C) by down-regulating miR-21. AMO-miR-21 alone effectively inhibited HL60 cell viability as measured by MTT assays and induced apoptosis as evaluated by flow cytometry, whereas AMO-miR-21 in combination with Ara-C enhanced HL60 cells to Ara-C-sensitivity and promoted Ara-C-induced apoptosis. Levels of miR-21 and its target PDCD4, quantified by real-time PCR, showed that expression of miR-21 was significantly decreased after AMO-miR-21 treatment. PDCD4 as a direct target of miR-21 in leukemic HL60 cells was confirmed by the dual-luciferase reporter assay. Our study suggests that AMO-miR-21 significantly sensitizes HL60 cells to Ara-C by inducing apoptosis and these effects of AMO-miR-21 may be partially due to its up-regulation of PDCD4. Therefore, exploiting synergistic effects between AMO-miR-21 and Ara-C might be an effective clinical strategy for leukemia chemotherapy.