High-density printed circuit boards (PCBs) with less than 50 μm/50 μm line/space has been developed. Since the spacing between adjacent signal lines becomes very narrow in the PCBs, the crosstalk from electromagnetic interference in adjacent lines becomes serious. The authors have proposed a magnetic method of reducing the crosstalk. PCBs with electroless-plated Ni-P alloy film on the signal lines were fabricated. From an experiment using a PCB-TEG with a 130 μm/50 μm line/space and 25-mm line length, crosstalk was suppressed throughout a wide frequency range. In using 6-μm thick Ni98P2.0 (wt.%) alloy film, the suppressed crosstalk was up to 12 dB around 1 GHz. However, the influence of transmission and reflection in the main signal line was very small.