The effect of optical vector potential in planar undulator is considered. The chosen electron beam is in two modes. For the Uniform distribution: The value for the space charge parameter is already equal to zero. The increasing value for optical vector potential leads to increasing in the output power gain. If the detuning parameter is larger than zero the values of optical vector potential will decrease and the output power gain will be decrease also. The maximum output power gain is satisfied at resonance condition. The second mode is energy spread parameter (Lorentzian distribution): In this case the output power gain is directly proportional with optical vector potential. Where the values of optical vector potential are decreases by increasing the values for energy spread parameter. This decreases in optical vector potential leads to decreases in output power gain. Therefore the output power gain in Uniform distribution is greater than the Lorentzian distribution.