A mouse melanoma model was set up with green fluorescent protein (GFP) expression in vivo. With the same energy, long- (1 ms) and short- (300 ns) pulsed electric fields were delivered to two melanomas injected into the same mouse. The tumor growth and green fluorescence were followed up to compare the different treatment efficacy of long and short pulses. After two days post treatment, short pulse-treated tumors showed a significantly lower tumor volume compared with long pulse-treated tumors (n=8, p<0.05). On 8 experimental animals, a short nanosecond pulsed electric field (nsPEF) had lesser or delayed effects on GFP quenching and greater effects in reducing tumor size. Short pulses produced by nsPEFs can cause melanoma regression with less effect on the plasma membrane.