<p> In this thesis, we are concerned with the outstanding problem of overbinding of a ^ particle in the spin-isospin saturated nuclear medium. The suppression effect in nuclear matter due to the repulsive core has been examined carefully and the single-particle-energy spectrum of the ^ particle is then derived. The two-channel formalism which takes account of the Σ^ conversion explicitly is used to study the binding of ^ in nuclear matter and in ^He5. With phenomenological ^N interactions which fit the low energy scattering data, it is shown that in this formalism the experimental binding energy of ^He5 can be reproduced.</p> / Thesis / Doctor of Philosophy (PhD)