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Grounded guard traces are increasingly utilized to mitigate crosstalk noise interference in printed circuit boards or packages. This paper discusses how the coupled striplines inserted guard trace with an open-stub affects the crosstalk noise. The guard trace with an open-stub, also called guard trace stub, seriously degrades the performance of the coupled striplines with respect to signal integrity, and the extra crosstalk noise occurs both at near- end and far-end for stripline structure. In addition, an analytical formula is proposed to approximate the maximum magnitude of extra crosstalk noise that is caused by a guard trace stub. To maintain the crosstalk noise induced by the active line and extra crosstalk noise induced by the guard trace stub on the victim line with equivalent amplitude is proposed to obtain the best design for maximum reduction ratio of crosstalk noise at both the near-and far-end. Based on the proposed method, a design curve for the geometric structures of coupled striplines inserted guard trace stub is constructed. Using the design curve, a maximum reduced ratio of crosstalk noise for coupled striplines inserted guard trace stub can be selected. Narrowing trace width of the guard trace stub is proposed to eliminate the crosstalk noise. The maximum reduction in crosstalk noise in the presented examples exceeded 50% in this paper. Finally, a favorable comparison between the time-domain simulation and measured results validates the proposed concept and analysis approach.