Abstract: A two-dimensional fractional Brown motion(fBm) model is presented for describing the rough ground surface,and we have analyzed the propagation effects of a fractal rough ground surface on the lightning-radiated electromagnetic field by using Barrick's theory and Wait's approximates. The results show that the rough land surface has no or little effect on the vertical electric field generated by the lightning first return stroke,however it has obvious effect on the field generated by the lightning suhsequent return stroke. The rise time of the field waveform increases with the increasing of the roughness,and the field peak decreases obviously. For example,when the ground conductivity is 0.1 S/m,the extra attenuation of the field peak and the extra risetime of the waveform caused by the roughness with a mean square height of 30 m are about 12% and 1.5 μs in its propagating 100 km,respectively. Therefore,we should take into account the errors in the accuracy and efficiency of the lightning location system detection caused by the rough propagation paths of the lightning field.