  In this paper we thermalize the type II superstrings in the GS formulation by
applying the TFD formalism. The thermal boundary conditions on the thermal
Hilbert space are obtained from the BPS $D$-brane boundary conditions at zero
temperature. We show that thermal boundary states can be obtained by
thermalization from the BPS $D$-branes at zero temperature. These new states
can be interpreted as thermal $D$-branes. Next, we discuss the supersymmetry
breaking of the thermal string in the TFD approach. We identify the broken
supersymmetry with the $\epsilon$-transformation while the
$\eta$-transformation is preserved. Also, we compute the thermal partition
function and the entropy of the thermal string.
