Abstract: As the velocity of signal switch becoming fast, the crosstalk in high-speed circuit design is becoming worse than ever. The performance of guard traces in reducing the crosstalk between microstrips is analyzed and simulated with FDTD method in this paper. The simulation results show that the far-end and near-end crosstalks are reduced largely and efficiently by the following useful measures: (1) The guard trace with several grounded vias should be inserted between microstrips and the distance between two adjacent vias should be less than propagating distance during RT/2, the half rising time of signal. (2) By obeying the smallest side-to-side distance routing rule and keeping the center-to-center distances between arbitrary two of three lines (microstrips and the guard trace) constant, the guard trace inserted should be as wide as possible.