  We study instanton effects on inclusive semileptonic $b \rightarrow u$ decay
using the heavy quark effective field theory and the operator product
expansion. The effect contributes not only to the hadronic matrix element but
also to the coefficient functions in the operator product expansion. We find
that the coefficient function is singular near the boundaries of the phase
space. In order to use perturbative QCD reliably, it is necessary to introduce
smearing near the boundary. However the instanton contribution to $b
\rightarrow c$ decay seems negligibly small.
