Three Parton Corrections in $B\to PP$ decays

The $1/m_b$ corrections from the three parton $q\bar{q}g$ Fock state of the final state light meson in $B\to PP$ decays are evaluated by means of a collinear expansion method. The impacts of these corrections on the $CP$ averaged branching ratios of the $B\to \pi K$ decays are analyzed.