Acutely isolated rat hippocampal CA3 pyramidal neurons were irradiated with a semiconductor laser of wavelength 670 nm and power 5 mW. The properties of transient outward potassium (K+) channel were investigated using the whole-cell patch clamp technique. The experiment revealed that low level laser reduced the amplitudes of IA in a voltage-dependent and reversible manner. The steady-state activation and inactivation processes of IA were significantly affected by laser irradiation. And the steady-state activation curve shifted towards positive potential direction, altering the slope factor of curve and increasing the half-activation voltage. However, the steady-state inactivation curve shifted towards negative potential direction, reducing the half-inactivation voltage without changing the slope factor. The results show that low level laser irradiation could change the characteristics of transient outward K+ channel. Accordingly, the firing of action potential and the physiological function of neurons were adjusted as a result of low level laser irradiation, which might contribute to the recovery and regeneration of injured neurons.