Crustal deformation and strain partitioning of oblique convergence between the Philippine Sea plate and the Eurasian plate in the southern Longitudinal Valley of eastern Taiwan were characterized, based on geodetic analysis of trilateration network and geological field investigation. The Longitudinal Valley fault, one of the most active faults on Taiwan, branches into two individual faults in the southern Longitudinal Valley. These two active faults bound the Plio-Pleistocene Pinanshan Conglomerate massif between the Coastal Range (the Luzon island arc belonging to the Philippine Sea plate) and the Central Range (the metamorphic basement of the Eurasian plate). A geodetic trilateration network near the southern end of the valley shows a stable rate of the annual length changes during 1983--1990. The strain tensors for polygonal regions (including triangular regions) of the Taitung trilateration network reveal that there are two distinct zones of deformation: a zone of shortening (thrusting) between the Pinanshan massif and the Central Range on the west and a strike-slip movement between the Pinanshan massif and the Coastal Range on the east. The analysis of a discontinuity model consisting of three rigid blocks separated by two discontinuities has been carried out. The results show that the deformation in this region can be characterized by two major faults. A reverse fault is located between the Plio-Pleistocene Pinanshan massif and the metamorphic basement of the Central Range, with a shortening rate of about 12 mm/yr in the direction N280 ¿E. A strike-slip fault is located principally along the river between the Pinanshan massif and island arc system of the Coastal Range with a purely strike-slip component of about 22 mm/yr in the direction N353 ¿E. The analysis of the geodetic data further suggests that substantial deformation (probably strike slip in type) occurs within the Pinanshan massif. Geological evidence of deformation in the Plio-Pliestocene Pinanshan Conglomerate includes regional folding, a conjugate set of strike-slip fractures at the outcrop scale, and morphological lineaments related to fracturing, all indicating that the Pinanshan massif is being deformed within a transpressive stress regime. Regional kinematic data indicate that a significant portion of the 82 mm/yr of motion between the Eurasian plate and the Philippine Sea plate is absorbed in the southern Longitudinal Valley by the decoupling of two distinct major faults. The geometry of the oblique convergence and the rheology of the rock units (the well-consolidated Plio-Pleistocene conglomerate and the sheared m¿lange formation) play the two important roles in the partitioning of crust deformation. ¿ 1998 American Geophysical Union