  The properties of nuclear matter are discussed with the relativistic
mean-field theory (RMF).Then, we use two models in studying the in-medium
properties of $\Theta^+$: one is the point-like $\Theta^*$ in the usual RMF and
the other is a K$\pi$N structure for the pentaquark. It is found that the
in-medium properties of $\Theta^+$ are dramatically modified by its internal
structure. The effective mass of $\Theta^+$ in medium is, at normal nuclear
density, about 1030 MeV in the point-like model, while it is about 1120 MeV in
the model of K$\pi$N pentaquark. The nuclear potential depth of $\Theta^+$ in
the K$\pi$N model is approximately -37.5 MeV, much shallower than -90 MeV in
the usual point-like RMF model.
