Histone deacetylase inhibitors (HDACIs) are currently in clinical trials partly due to their potent anti-angiogenic effects. However, the detailed mechanism of their action is unclear. Here, we observed that several HDACIs (TSA, SB, Apicidin, and VPA) dramatically decreased HIF-1alpha protein level and transcriptional activity of HIF-1 in human and mouse tumor cell lines. Furthermore, class I HDACs, HDAC1 and 3 enhanced HIF-1alpha stability and HIF-1 transactivation function in hypoxic conditions. In addition, immunoprecipitation and in vitro binding assays revealed that HDAC1 and 3 directly bind to the oxygen-dependent degradation domain of HIF-1alpha. Collectively, these results suggest that HDAC1 and 3 are considered as a positive regulator of HIF-1alpha stability via direct interaction and may play an important role in HIF-1-induced tumor angiogenesis.