The Transient Electromagnetic (TEM) method was used to map geoelectric structures beneath the Longitudinal Valley, eastern Taiwan. Coincident loop configurations with square transmitting loops 90 to 200 m on each side were used for this survey. More than 300 soundings were conducted in this valley during 1991-1993. Field data were inverted by the adaptive nonlinear least-squares technique. The sounding depths were more than 200 m beneath the Longitudinal Valley. A 3D resistivity image of the Longitudinal Valley was obtained. The resistivity patterns in the western part of the Longitudinal Valley are apparently higher and relatively more heterogenous, in contrast with a resistivity which is on average lower and relatively less homogenous in the eastern part of the Longitudinal Valley. This feature correlates well with the surface geology of the area. Morever, the TEM soundings show that one predominant NNE-trending electrical transition in the Longitudinal Valley agrees well with the known suture trace on surface between the Philippine Sea and the Eurasian plate. On the basis of the resistivity pattern at different depths, the crust in this area is subject to a horizontal compressional stress oriented preferentially along the NWW-SEE direction. That the Longitudinal Valley is undergoing severe compression by the Philippine Sea plates can also be recognized. However, due to the limited depth of exploration, deeper investigations are needed to completely resolve the actual tectonic features.