Abstract: Objective The aim is to study the discharge mechanism and material transfer mechanism of electric-spark deposition (ESD) in surface contact state. Methods The automatic ESD experiments are carried out with different cylindrical electrode diameters. The transfer efficiency, deposition efficiency, discharge waveform of ESD and surface morphology, cross-section morphology, surface roughness of the deposition layer are analyzed in different electrode diameters. Results The results show that the transfer efficiency and deposition efficiency increase with the increase of electrode diameter, and the surface roughness of deposition layer also increases accordingly when the cylindrical electrode is deposited vertically with the workpiece surface. When the electrode diameter exceeds a certain value, both transfer efficiency and deposition efficiency as well as surface roughness exhibit a decreasing trend. A uniform deposition layer can be obtained when the electrode diameter is less than 4 mm, however, when the electrode diameter is more than 5 mm, the discharge and deposition are mainly concentrated on localized area of workpiece surface leading to an uneven thickness in the deposition layer. Conclusion The increase of electrode diameter results in the increase of contact area between electrode end face and workpiece surface and the increase of contact points, which reduces the discharge current density at each contact point during the deposition process, affects the material transfer process and reduces the sparks and splatters.