A new method, based on zero point of longitudinal electric field, was used to determine the spatial resolution of internal and external electro-optic (EO) probing systems. An interdigital structure electrode was fabricated on GaAs substrate to examine the spatial resolution of internal and external EO probing system. Considering the air gap between the electrode under test and the probe tip, the result of EO probing was simulated which is well accorded with experiment. The result indicates that spatial resolution of less than 0.5 and 1 m are obtained in internal and external EO probing system respectively.