  We show that the amplitudes for B decays into two light mesons at leading
order in soft-collinear effective theory are factorized to all orders in
alpha_s. We construct gauge-invariant four-quark operators by employing the
effective theories SCET_I for sqrt{m_b \Lambda} <\mu<m_b, and SCET_II for \mu
<sqrt{m_b \Lambda} with Lambda ~0.5 GeV. These operators do not allow gluon
exchanges between different sectors of the current and the matrix elements are
reduced to the products of current matrix elements. The spectator interactions
are also factorized at leading order in SCET and to all orders in alpha_s.
