In this work, the work function variation (WFV) and process variation effect (PVE) on 5 nm node gate-all-around (GAA) silicon 3D stacked nanowire field-effect transistor (NWFET) devices are studied through technology computer-aided design (TCAD) simulations. The WFV effect on 3D stacked NWFETs leads to stronger immunity compared to the same effect on single NWFETs. On the other hand, the 3D stacked NWFET is significantly affected when each stack is varied due to PVE. As the PVE becomes increasingly more serious, it is important to analyze the degree of variability of each stack in a NWFET. In addition, we closely investigate the WFV effect and device an accurate guideline with regard to the NW diameters of single and 3D stacked NWFETs affected by the PVE.