We aimed to directly convert adult human dermal fibroblasts (aHDFs) into functional endothelial cells (ECs). Lentiviral vectors encoding endothelial transcription factors (TFs) were constructed. We examined whether five TFs (FOXO1, ER71, KLF2, TAL1, and LMO2) used for the generation of mouse induced ECs (iECs) could convert the aHDFs into human iECs. Twenty-eight days after transduction with lentiviral constructs, 32.1 ± 5.1% cells expressed vascular endothelial (VE)-cadherin. Factor screening revealed that only three factors (3F: ER71, KLF2, and TAL1) were necessary to induce VE-cadherin (+) cells (49.4 ± 3.5%). However, whole transcriptome sequencing showed that VE-cadherin (+) cells were not completely reprogrammed. Mature iECs double-positive for VE-cadherin/Pecam1 (DP cells) with a cobblestone appearance were obtained at a frequency of only 5.1 ± 0.6%. Using whole transcriptome analysis, the potential factors that could block the conversion were screened. Among candidates TWIST1-knockdown enhanced efficiency of conversion. Rosiglitazone, an inhibitor of epithelial-mesenchymal transition (EMT), also improved the conversion efficiency. Moreover, a 2nd second-stage conversion process, in which VE-cadherin (+) cells were incubated for additional two weeks, further enhanced the efficiency. The final protocol for 6 weeks yielded a conversion rate of 19.6 ± 3.0% iECs, defined by DP cells depicting the nature of mature ECs in various analyses. Further analyses revealed that the genetic and epigenetic profiles of iECs resembled those of functional ECs. Collectively, aHDFs can be converted into functional ECs through the transduction of ER71, KLF2, and TAL1, combined with two EMT inhibitors (siTWIST1 and rosiglitazone), followed by 2nd stage conversion.