Acute myeloid leukemia (AML) requires new therapies on the molecular level. Downregulation of heme oxygenase-1 (HO-1) by gene silencing improves the sensitivity of tumor cells to chemotherapy drugs and promotes apoptosis. For the first time, we verified that endoplasmic reticulum and mitochondrial apoptotic pathways were activated by small interfering RNA that targeted-silenced the expression of HO-1 in AML-M2 Kasumi-1 cells. Ca2+ was prone to accumulation and reactive oxygen species were easily generated, while mitochondrial transmembrane potential was reduced. Thus, cytochrome c was released from mitochondria to the cytoplasm and caspases were activated for the following cascade to facilitate apoptosis.