Using the technology computer-aided design (TCAD) tool, the impact of work function variation (WFV) in stacked nanowire field effect transistor (stacked NWFET) is investigated. To study the WFV-induced device performance variation, the metal grain granularity with the work function values and probabilities, which are corresponding to the gate material in the high-k/metal gate of stacked NWFET, is formed and applied in the stacked NWFET. It turned out that, in the gate-all-around nanowire device structure, the WFV-induced threshold voltage (VT) variation can be effectively suppressed because the effective grain size around the channel region is smaller than the actual grain size. To explore the WFV-induced VT variation in stacked NWFET, the number of stacks (i.e., single, 2-/3-stacked NWFET) and distance in-between nanowires (i.e., 10.5, 13.5, 16.5, 19.5, and 22.5 nm) are quantitatively varied. In stacked NWFET, the WFV-induced VT variation is effectively suppressed as the number of stacked nanowires increases. On the other hand, it is revealed that the distance in-between nanowires in stacked NWFET has nothing to do with the WFV-induced VT variation.