The this paper deals with the analysis of power distribution network (PDN) impedance and simultaneous switching output buffer (SSO) noise for a 3D system-in package (SiP) with 4k-IO widebus structure. The 3D SiP consisted of 3 stacked chips (a memory chip on the top, Si interposer in the middle, and a logic chip) and an organic package substrate. More than 4096 of through silicon vias (TSV's) were formed to the silicon interposer and the logic chip. The PDN impedance for each chip was extracted by using XcitePI (Sigrity Inc.). Then, the PDN impedance for the organic package substrate was extracted by using SIwave (Ansys Inc.). Finally, the total PDN impedance was synthesized to estimate the power supply disturbance due to the anti-resonance peak.