Endothelial cell (EC) is important tissue that has high plasticity in response tovascular millieu change. However, its plasticity turn EC lead to its dysfunction and contributein several disease patomechanism. An inflammation agent such as TNF-α can induce ECdysfunction via endoplasmic reticulum stress (ERS) pathway. Targeting ERS was new approachin vascular biology to inhibit EC dysfunction. One of its biomarker is activating transcriptionfactor 6 (ATF6). However, ERS role in endothelial cells is still poorly understood. Hence weperformed in vitro experiment using phenyl butyrate acid as ERS inhibitor to TNF-α -inducedATF6 expression in human vein derived endothelial cells. We measured ATF6 expression inendothelial cells as ERS biomarker and use phenyl butyrate acid (PBA) as potent selective ERSinhibitor to block its pathway. The early result shows that PBA decrease translocated ATF6expression in endothelial culture. From the result, it can been concluded that PBA has role indecrease endoplasmic reticulum stress in endothelial cells.Keywords: Endothelial cell, patomechanism, phenyl butyrate acid