  D0-branes moving in a constant antisymmetric C field are found to be
described by quantum mechanics of the supersymmetric matrix model with a
similarity transformation. Sometimes this similarity transformation is singular
or ill-defined and cannot be ignored. As an example, when there are
non-vanishing C_{-ij} components, we obtain the theory for Dp-branes which is
effectively the noncommutative super Yang-Mills theory. We also briefly discuss
the effects of other non-vanishing components such as C_{+ij} and C_{ijk}.
