  We study the KN interactions in the I(J^{\pi})=0(1/2^-) and 1(1/2^-) channels
and associated exotic state \Theta^+ from 2+1 flavor full lattice QCD
simulation for relatively heavy quark mass corresponding to m_{\pi}=871 MeV.
The s-wave KN potentials are obtained from the Bethe-Salpeter wave function by
using the method recently developed by HAL QCD (Hadrons to Atomic nuclei from
Lattice QCD) Collaboration. Potentials in both channels reveal short range
repulsions: Strength of the repulsion is stronger in the I=1 potential, which
is consistent with the prediction of the Tomozawa-Weinberg term. The I=0
potential is found to have attractive well at mid range. From these potentials,
the $KN$ scattering phase shifts are calculated and compared with the
experimental data.
