  We explore a possibility of optimization of the method of determination of
the top quark mass from $M_{b\ell}$ distribution in semi-leptonic decays $t \to
b \ell\nu$ at LHC and a future linear collider (LC). We discover that the
systematic and statistical errors of $M_t$ determination can be diminished if
considering the high moments over the distribution. In the case of LHC this
allows one to reduce more than in twice the errors, and in the case of LC to
approach to the precision expected at studying the threshold scan of the total
cross-section $e^+e^- \to t\bar t$.
