The transient electromagnetic (TEM) method was used to map plate boundaries in the Longitudinal Valley (LV), Taiwan. The SIROTEM in-loop configuration with square transmitting loops 90 to 200 m on a side were used for this survey. More than 350 soundings were conducted in the valley. The field data were inverted by a smooth model inversion, and the soundings resolved more than 300 m beneath the LV. A set of geoelectric maps containing horizontal slices at different depths and vertical profiles from north to south along the LV have been constructed based on these inversion results. Based on the TEM sounding results, one predominant electrical discontinuity can be recognized in the LV that agrees well with the known suture trace on the surface between the Philippine Sea and the Eurasian plates. Moreover, the fact that the sedimentation trend thickened southward in the LV supports the geologic viewpoint that the LV is undergoing severe compression closure action; the northern part of the valley seems to have been closed already, whereas the southern part has not.