To achieve the pre-visual detection on potassium (K) stress，the microelectrode technique was used to measure the vacuolar membrane potential of cucumber leaves under four different levels of K-treatment. Kdeficiency made the vacuolar membrane potential of cucumber leaves more negative, whereas K-excess made the vacuolar membrane potential less negative. An obvious decrease of vacuolar membrane potential could be observed after 2d of severe K-deficiency plants. An obvious decrease of the vacuolar membrane potential could be observed after 4d of moderate K-deficiency plants. Conversely, an obvious increase of the vacuolar membrane potential could be observed after 2d of K-excess plants .The measurement of vacuolar membrane potential in cucumber leaf could be used for pre-visual detection of Kstress 10d before the appearance of visually morphological changes. All of these showed that the vacuolar membrane potential measurement may be used as an early diagnosing tool for K-stress in cucumber plants. It should be pointed out that the active transport of K+ does not contribute to the primary generation of membrane potential but is contingent upon an electrochemical proton gradient to furnish the required energy. Ultimately, the establishment of this gradient by the activity of the plasma membrane H+ pumping ATPase is the means by which plant cells become electrically polarized.