  We study an ${\cal N}=1$ supersymmetric Yang-Mills theory defined on
$M^4\times S^1$. The vacuum expectation values for adjoint scalar field in
vector multiplet, though important, has been overlooked in evaluating one-loop
effective potential of the theory. We correctly take the vacuum expectation
values into account in addition to the Wilson line phases to give an expression
for the effective potential, and gauge symmetry breaking is discussed. In
evaluating the potential, we employ the Scherk-Schwarz mechanism and introduce
bare mass for gaugino in order to break supersymmetry. We also obtain masses
for the scalars, the adjoint scalar, and the component gauge field for the
$S^1$ direction in case of the SU(2) gauge group. We observe that large
supersymmetry breaking gives larger mass for the scalar. This analysis is
easily applied to the $M^4\times S^1/Z_2$ case.
