In this paper, we theoretically study the quantum size effects on the electronic transmission and current density of the electrons in GaAs/AlGaAs resonant tunneling diodes by solving the coupled equations Schrodinger-Poisson self-consistently. It is found that the resonant peaks of the trans-mission coefficients shift towards the lower energy regions as the applied bias voltage increases. Our results indicate that the transmission coefficient depends strongly on the variation of the thickness of collector and emitter. We also study the effect of the doping concentration located in the emitter and collector regions on the transmission and current density. We found that the dop-ing concentration can greatly affect the transmission coefficient and the current density; in partic-ular it increases the peak of the current density and displaces the position of the maxima of the current dependence on the applied bias voltage.