The J-U relation of the KTP crystal along its z-axis under low DC field deviates from linearity, which is different from that of the crystal along the other two principle axis. The resistance of the crystal along its z-axis increases with the time under high DC stress and the crystal becomes dark. The dark color can be removed by annealing. Electrode reactions were also observed. The phenomena are related to the easy movement of ions along the z-axis of the KTP crystal.