  The electroweak phase transition (EWPT) in the secluded-$U(1)'$-extended MSSM
(sMSSM) is studied. Using the effective potential at zero and finite
temperatures, we search for the non-MSSM-like EWPT in which the light stop mass
is larger than the top quark mass. Scanning the parameters relevant to the
EWPT, the upper limits of the Higgs boson masses, which are consistent with the
strong first order EWPT, are derived. For the lightest CP-even and -odd Higgs
bosons, we find $m_{H_1}<160$ GeV and $m_{A_1}<250$ GeV, respectively. In the
sMSSM, the tree-level CP violation is possible by the complex soft
supersymmetry breaking masses. It is observed that such a CP-violating effect
does not spoil the strong first order EWPT for the typical parameter sets.
