  We give an interpretation of the non-supersymmetric heterotic theories as the
supersymmetric heterotic theories on Melvin background with Wilson lines. The
coincidence of the partition functions are shown for most of the
non-supersymmetric theories. We also discuss the tachyonic instability in terms
of the non-trivial background fields. The behavior in the strong coupling
region is investigated by using the duality relations of the supersymmetric
theories.
