We present O(alpha_s^2) corrections to the decay t–>bW in the limit
of a very large top quark mass, m_t » m_W. We find that the
O(alpha_s^2) effects decrease the top quark decay width Gamma_t by
about 2%: Gamma_t = Gamma_0(1-0.8alpha_s(m_t)-1.7alpha_s^2). The
complete corrections are smaller by about 24% than their estimate
based on the BLM effects O(beta_0 alpha_s^2). We explain how to
compute a new type of diagrams which contribute to Gamma(t –> bW) at
the O(alpha_s^2) level.