  The effect of a unidirectional periodic potential on the orientation of the
stripe state is studied for the two-dimensional electron system at half-filled
high Landau levels. By considering a quantum well with two electric subbands,
it is found that the stripe is parallel to the external potential for weak
modulation and is orthogonal for strong modulation. In the intermediate range,
the orientation of the stripe changes from orthogonal to parallel as the
electron density is increased. This result explains the recent experiment
performed by J. Zhu {\it et al} that the anisotropy axis at half-filled high
Landau levels rotates by $90^0$ by increasing the electron density. It also
supports the suggestion that the stripes is pinned by the native surface
morphology at the interface of the heterojunction.
