In this study, electrical discharge alloying process is used to modify the surface of ferritic spheroidal graphite (SG) cast iron. The surface modification of SG cast iron with electrical discharge alloying is used to examine the effect of discharge current on the thickness, hardness and surface roughness of the surface modified layer while the pulse duration, duty factor, electrode polarity (negative electrode) and the electrode composition (Fe-75mass%Si) are fixed. Experiment results reveal that the thickness of the surface modified layer tends to increase as the discharge current increases. In addition, the microstructure of the layer consists of fine dendritic crystals and tiny graphite particles. The results of EPMA compositional analysis indicate that the silicon element was contained in the electrode effectively dissolved in the alloying layer of SG cast iron and the content of the dissolved electrode elements decreases gradually from the discharge surface to substrate bulk. The Si concentrations and micro-hardness of the alloying layer also increase with the discharge current. The results of X-ray diffraction on the Si alloying layer show that α phase, ε phase and metastable phase of FeSiC are the main compositional phases.