The 2008 5.2 Mt. Carmel earthquake is the largest earthquake in the last 50 yrs in southeastern Illinois, near the north termination of the north‐northeast‐trending Wabash Valley fault system (WVFS). The earthquake shows almost pure strike‐slip focal mechanism, but it is still uncertain which nodal plane (NP) is the ruptured fault plane. To resolve the fault plane, we determine rupture directivity of the earthquake via the relative centroid method. We begin with inverting the point‐source solution (strike 297°/dip 84°/rake 1° for NP1, strike 206°/dip 89°/rake 173° for NP2, and centroid depth 16 km) and then determine the relative location between the centroid and hypocenter via regional waveform fitting. Two M 4+ aftershocks are used as reference events, and the waveform time shifts of reference events with respect to the 1D velocity model are used to calibrate the path effects. The results show that the Illinois mainshock ruptured to east‐southeast along the 297° NP for about 2–3 km, consistent with relocated aftershock distribution, and we infer that the sinistral causative fault connects the north‐northwest‐trending La Salle anticlinal belt and the north‐northeast‐trending WVFS.