AIM: To study the effect of microelement powder (MP) on membrane potential of vascular endothelial and smooth muscle cells of rats in order to elucidate the mechanism of microcirculation improvement by MP. METHODS: Cultured pulmonary vascular endothelial cells (EC) and aortic smooth muscle cells (SMC) of rats and detecting the changes of cellular membrane potentials by using potential-sensitive fluorescent probe and laser jet confocal microscope. RESULTS: MP hyperpolarized SMCs significantly. Glybenclamide (2 ?mol/L), a blocker of K ATP channel, which had no effect on membrane potential of SMCs, reversed the hyperpolarization of MP completely; MP hyperpolarized ECs slightly, but the effect was unaffected by glybenclamide. CONCLUSION: MP hyperpolarizes SMCs by activating K ATP channels and leads to dilation of microvessels and improvement of microcirculation.