Abstract: We apply Okadas method to calculate changes of static Coulomb failure stress on the largest aftershock (Ms=5.6) failure plane caused by the 2008 M,6.1 Panzhihua-Huili earthquake,and find that the stress change on the largest aftershock failure plane caused by the main shock is 0.37MPa，which is larger than the typical 0.01MPa stress change necessary for triggering earthquakes.Therefore,the largest aftershock was probably triggered by the Coulomb failure stress change caused by the main shock.