The cw electro‐optic probing technique is for the first time proposed to detect the three‐dimensional internal field distribution in linear electro‐optic material like GaAs. By changing the incident angles and positions of the probing beam, sufficient information of the electric field distribution is included in the phase retardation of the probing beam. If certain conditions on the probing beam are satisfied, a very simple linear relation between phase retardation and each field component can be found and the whole problem becomes not only mathematically solvable but experimentally feasible. Finally, a three‐dimensional computer simulation is undertaken to illustrate the relation between field distribution and detected electro‐optic signal.