Power/ground partitioning has been used, to supply multivoltage levels and to isolate power/ground noise in high-speed multilayer printed circuit boards. However, the partitioning of the power/ground plane breaks the current return path of the signal current through either the power plane or the ground plane, which causes undesired effects such as signal distortion, crosstalk, and radiation. To control and suppress these undesired effects, we should understand the electromagnetic mechanism associated with them. In this paper, the mechanism of the reflection and the transmission of the signal by the slotted power/ground plane is well understood through an analysis of measurements based on time-domain reflectometry. Considering the propagation of a slot wave through the slot line on the power/ground plane, we have successfully explained the changes of the transmitted and reflected waveforms. Furthermore, we have numerically and experimentally investigated the effects of the power/ground partitioning on the radiated emission in various structures. Finally, it is confirmed that the employment of a stitching capacitor on the power/ground slot suppresses the signal distortion and the radiated emission significantly. When the size and the location of the stitching capacitor are designed, there should be a compromise between the noise isolation and the guarantee of the return current path, with considering the resonance frequencies of planes by the capacitor.