We have collected a large number of accelerograms recorded by the Taiwan Strong Motion Instrumentation Program (TSMIP) stations to study the thickness variations of Quaternary alluviums beneath Lan-Yang Plain, Taiwan, using an Sp converted wave. The estimated thicknesses of the Quaternary sediments inferred by the travel-time difference of S and Sp waves are between 200 and 1400 m and become thicker toward the northeast. In general, our resulting features of the time difference of arrivals between Sp and S waves are consistent with the previous studies on thickness variations of the Quaternary alluviums beneath Lan-Yang Plain assuming the converting point is at the unconsolidated Quaternary alluvial sediments–Miocene basement interface. Our study suggests that this technique of using P-S converted phases could be applied to the other populated basins or plains in the Taiwan region based on its dense coverage of the TSMIP stations and high seismic activity. This technique is simple and time effective and can be used to determine the general characteristics of velocity/thickness structure of a study area.