We explore regional stress states in the vicinity of the rupture area of the 1999 Chi-Chi, Taiwan, earthquake by reconciling data from the Taiwan Chelungpu-Fault Drilling Project (TCDP) in-situ measurements and earthquake focal mechanisms. Given the background deviatoric stress in the range of 10-50. MPa and the horizontal NW-SE directed maximum principal stress axis, the predicted fault types show strike-slip and normal faulting near the coseismic surface rupture and thrust and strike-slip faulting in central Taiwan. Such predictions are able to fit TCDP in-situ observations in a local scale and aftershock earthquake focal mechanisms in a regional scale. Additionally, the proposed stress state explains remarkable rotations of the maximum stress axes observed near the northern segment of the Chelungpu Fault. This result provides key information for forecasting of consequent earthquakes and evaluation of focal mechanisms after the occurrence of a large earthquake.