  We construct bosonic string theories, RNS string theories and heterotic
string theories on flat supermanifolds. For these string theories, we show
cancellations of the central charges and modular invariance. Bosonic string
theories on supermanifolds have dimensions (D_B,D_F)=(26,0),(28,2),(30,4),...,
where D_B and D_F are the numbers of bosonic coordinates and fermionic
coordinates, respectively. We show that in type II string theories the one loop
vacuum amplitudes vanish. From this result, we can suggest the existence of
supersymmetry on supermanifolds. As examples of the heterotic string theories,
we construct those whose massless spectra are related to N=1 supergravity
theories and N=1 super Yang-Mills theories with orthosymplectic supergroups on
the bosonic flat 10 dimensional Minkowski space. Also, we construct D-branes on
supermanifolds and compute tensions of the D-branes. We show that the number of
fermionic coordinates contributes to the tensions of the D-branes as an inverse
power of the contribution of bosonic coordinates. Moreover, we find some
configurations of two D-branes which satisfy the BPS-like no-force conditions
if \nu_B - \nu_F = 0,4 and 8, where \nu_B and \nu_F are the numbers of
Dirichlet-Neumann directions in the bosonic coordinates and in the fermionic
coordinates, respectively.
