Endoplasmic reticulum (ER) stress in endothelial cells often leads to endothelial dysfunction which underlies the pathogenesis of cardiovascular diseases. Paeonol, a major phenolic component extracted from Moutan Cortex, possesses various medicinal benefits which have been used extensively in traditional Chinese medicine. The present study investigated the protective mechanism of paeonol against tunicamycin-induced ER stress in isolated mouse aortas and human umbilical vein endothelial cells (HUVECs). Vascular reactivity in aorta was measured using a wire myograph. The effects of paeonol on protein expression of ER stress markers, reactive oxygen species (ROS) production, nitric oxide (NO) bioavailability and peroxisome proliferator-activated receptor δ (PPARδ) activity in the vascular wall were assessed by Western blot, dihydroethidium fluorescence (DHE) or lucigenin enhanced-chemiluminescence, 4-amino-5-methylamino-2′,7′-difluorofluorescein (DAF-FM DA) and dual luciferase reporter assay, respectively. Ex vivo treatment with paeonol (0.1 μM) for 16 h reversed the impaired endothelium-dependent relaxations in C57BJ/6J and PPARδ wild type (WT) mouse aortas following incubation with tunicamycin (0.5 μg/mL). Elevated ER stress markers, oxidative stress and reduction of NO bioavailability induced by tunicamycin in HUVECs, C57BJ/6J and PPARδ WT mouse aortas were reversed by paeonol treatment. These beneficial effects of paeonol were diminished in PPARδ knockout (KO) mouse aortas. Paeonol increased the expression of 5′ adenosine monophosphate-activated protein kinase (AMPK) and PPARδ expression and activity while restoring the decreased phosphorylation of eNOS. The present study delineates that paeonol protects against tunicamycin-induced vascular endothelial dysfunction by inhibition of ER stress and oxidative stress, thus elevating NO bioavailability via the AMPK/PPARδ signaling pathway.