Recently, high-density printed circuit boards (HD-PCBs) with less than 50 mum/50 mum line/space has been developed. In the HD-PCB, since the spacing between adjacent signal lines becomes very small, the crosstalk electromagnetic interference among adjacent lines becomes serious. In general, in order to suppress the crosstalk, the ground line is located at the spacing between signal lines because of the decrease of mutual capacitance among adjacent signal lines. The authors have proposed a magnetic method for reducing the crosstalk. The composite magnetic material was filled in the spacing between signal lines instead of the ground line. The magnetic composite is composed of Fe-Si-B-Cr amorphous particles with a mean diameter of 6 mu m and epoxy resin. From the experiments using the PCB-TEG with a 130 mum/50 mum line/space and 25-mm line length, the crosstalk was suppressed in the wide frequency range by using the magnetic composite. In the case of using 51 vol.% amorphous composite, the crosstalk suppression was up to 20 dB around 1 GHz. The influence of the magnetic composite on the transmission and reflection in the main signal line was very small. Therefore, the proposed method is effective for crosstalk suppression without influence on the signal transmission.