  The weak $\Lam N\to NN$ transition is studied in the valence quark model
approach. The momentum transfer for this transition is so large that the
short-distance two baryon dynamics must be taken into account. The two baryon
system is described in the quark cluster model and the weak transition
amplitude is calculated by evaluating the matrix elements of the effective weak
$\Delta S= 1$ hamiltonian. The results indicate some qualitative differences
when compared with those in conventional meson-exchange calculations.
Especially, we conclude that contributions of the $\Delta I={3\over2}$
transition are significant and that the discrepancy in the $n-p$ ratio between
theory and experiment could be resolved by including the direct-quark
processes.
