We carried out a seismotectonic study on the 1998 Ruey-Li, Taiwan earthquake. This ML=6.2 event occurred in a close neighborhood to and about one year before the MW=7.6 disastrous Chi-Chi, Taiwan earthquake of 1999. Both the Ruey-Li and the Chi-Chi events share a common regional stress system. From the relocated hypocenters, focal mechanisms and stress inversion of the Ruey-Li sequence, we find that the spatial distribution of aftershocks forms two distinct groups, one is consistent with a planar thrust, the other gives a peculiar 3km ¡_ 5km ¡_ 15km of nearly vertical columnar distribution made principally of left-lateral strike-slip faulting. A regional block rotation model is proposed to explain the rupture process; this model is supported by the well-documented GPS data.