Periodontal ligament is considered a rich source of mesenchymal stem cells for pulp regeneration. This study investigated the effect of iloprost, a prostacyclin analog, on cell proliferation and the expression of angiogenesis-related genes in human periodontal ligament cells (hPDLs) in vitro. The hPDLs were treated with 10-10 M iloprost for 1, 6 and 24 h. Cell proliferation was determined by an MTT assay. Vascular endothelial growth factor (VEGF), alpha-1 type I collagen (COL1), and basic fibroblast growth factor (bFGF) mRNA expression were determined by semi-quantitative and qPCR. ELISA was employed for assessment of VEGF expression. Immunofluorescence staining for COL1 protein expression was performed. A prostacyclin receptor (IP) antagonist was used to verify the signaling pathway. The Kruskal-Wallis and Tukey significant difference tests were used to analyze the results. From the results, iloprost treatment did not affect cell morphology or proliferation. Iloprost induced the upregulation of VEGF and COL1 mRNA levels as shown by PCR. The effect of iloprost on bFGF mRNA expression was not observed. The immunofluorescence assay revealed that COL1 protein expression was increased in the iloprost groups. Pretreating the hPDLs with the IP antagonist significantly suppressed the enhancing effect of iloprost on VEGF and COL1 mRNA expression and suppressed COL1 protein expression. In conclusion, iloprost promoted mRNA and protein expression of VEGF and COL1, but not of bFGF in hPDL cells. The increased effect of iloprost was abolished by IP receptor antagonist pretreatment. Iloprost might be a promising agent in dentin-pulp regeneration.