Protease-activated receptors (PARs) are multifunctional G protein-coupled receptors. Among the four existing PARs, PAR4 is preferentially expressed in the human lung tissue. However, the function of PAR4 has not been defined in the lung endothelial cells. Because PAR1-mediated cellular effects are deeply related to the morphological changes, we focused on the actin fiber and p38 mitogen-activated protein kinase (MAPK) signaling involved in actin polymerization to elucidate the role of PAR4. RT-PCR and Western blot analyses identified PAR4 expression in human pulmonary artery endothelial cells and in human microvascular endothelial cells from lung. We then examined the changes in actin fibers in endothelial cells treated with PAR4-activating peptide. PAR1-activating peptide was used for comparison. Activation of PAR4 and PAR1 by their corresponding peptides induced actin fiber formation; however, the actin filaments were broadly bundled in PAR4 as compared with the ringlike actin filaments in PAR1 activation. Correspondingly, the magnitude of p38 MAPK phosphorylation was different between cells treated with PAR4 and PAR1, with PAR4-activating peptide showing a significantly higher sensitivity to p38 MAPK inhibitor, SB203580. Taken together, these results demonstrate that activation of PAR4 results in the formation of actin fiber distinct from that by PAR1 activation, suggesting PAR4 may play specific roles in the lung endothelial cells.