Novel experimental characterization method and circuit modeling for frequency-variant transmission lines are presented. Experimental test patterns are designed and fabricated by using a BGA package process. They are characterized and modeled by using time-domain TDR/TDT wave measurements and frequency-domain s-parameter measurements. Then taking the frequency-variant effects into account, a transmission line circuit model is developed, followed by the s-parameter-based model verification. Thereby, it is shown that the conventional constant-model-parameter-based circuit model may not be accurate enough to verify the signal integrity of high-performance integrated circuits.