Nitric oxide (NO) produced by endothelial NO synthase (eNOS) plays an important role in vascular functions, including vasorelaxation. We here investigated the pharmacological effect of the natural product syringaresinol on vascular relaxation and eNOS-mediated NO production as well as its underlying biochemical mechanism in endothelial cells. Treatment of aortic rings from wild type, but not $eNOS^{-/-}$ mice, with syringaresinol induced endothelium-dependent relaxation, which was abolished by addition of the NOS inhibitor $N^G$-monomethyl-L-arginine. Treatment of human endothelial cells and mouse aortic rings with syringaresinol increased NO production, which was correlated with eNOS phosphorylation via the activation of Akt and AMP kinase (AMPK) as well as elevation of intracellular $Ca^{2+}$ levels. A phospholipase C (PLC) inhibitor blocked the increases in intracellular $Ca^{2+}$ levels, AMPK-dependent eNOS phosphorylation, and NO production, but not Akt activation, in syringaresinol-treated endothelial cells. Syringaresinol-induced AMPK activation was inhibited by co-treatment with PLC inhibitor, $Ca^{2+}$ chelator, calmodulin antagonist, and $CaMKK{eta}$ siRNA. This compound also increased eNOS dimerization, which was inhibited by a PLC inhibitor and a $Ca^{2+}$-chelator. The chemicals that inhibit eNOS phosphorylation and dimerization attenuated vasorelaxation and cGMP production. These results suggest that syringaresinol induces vasorelaxation by enhancing NO production in endothelial cells via two distinct mechanisms, phosphatidylinositol 3-kinase/Akt- and PLC/$Ca^{2+}/CaMKK{eta}$-dependent eNOS phosphorylation and $Ca^{2+}$-dependent eNOS dimerization.