Although the ion motions are very vital to the plasma immersion ion implantation, the electron motions also play the key role to sheath formation and electric filed distribution. However, only ion motion was calculated in the conventional theoretical model for PIII process, the influence of electron motion was ignored. In order to describe the ion sheath formation of the PIII process, the electron motion equation was derived based on the some assumptions, and analytical solution of the electron motion was derived in the planar target. The information related with the time to form the ion sheath, the ion sheath thickness and the electrical field distribution can be found from the analytical solution. Especially, the ion formation sheath time is found to be equal to 2.078 Ln frac12 omegape -1, where Ln is the normalized length of process chamber and omegape is the plasma electron frequency, which much larger than common estimated value omegape -1. The computer simulation of the ion sheath formation was also carried out with OOPIC Pro in order to verify the analytical analyses of the ion sheath formation. The simulation results show that the simulation is in accordance with the theoretical analyses.