Hypoxia-inducible factor (HIF) is a heterodimer composed of HIF-α and -β, and to date, three HIF-α subunits, HIF-1α, -2α, and -3α, have been identified. Among these HIF-α subunits, HIF-1α represses peroxisome proliferator activator γ2 gene expression and then inhibits adipogenesis, and HIF-2α is induced during adipose differentiation and functions as a positive regulator of adipogenesis. We here found that like HIF-2α, HIF-3α was induced during 3T3-L1 adipose differentiation. Reporter gene analysis revealed that HIF-2α enhanced the promoter activity of the 5′-flanking region of the mouse HIF-3α gene (nucleotides −2710 to +56), while HIF-1α had no substantial effects on the promoter activity. These results suggested that HIF-2α, which was induced during adipogenesis, might regulate the HIF-3α gene expression. Furthermore, the 5′-deletion analysis revealed that the sequence between −251 and −228 in mouse HIF-3α promoter was essential in response to HIF-2α. We further examined the effect of ectopic expression of HIF-3α in 3T3-L1 cells on adipose differentiation and found that ectopic expression of HIF-3α at the early stage of differentiation induced the expression of several kinds of adipocytes-related genes and enhanced adipogenic potential. HIF-3α, which is induced by HIF-2α, might function as an accelerator of adipogenesis.