  In the minimal supersymmetric standard model (MSSM), a strongly first-order
electroweak phase transition (EWPT) is only possible in a confined parameter
region where one of the scalar top quarks is lighter than the top quark and the
other one is as heavy as the SUSY breaking scale. If the MSSM is enlarged to
accommodate vector-like quarks and their superpartners, we find that the
strongly first-order EWPT is possible without requiring light scalar top quark
at the one-loop level, in the limit where the lightest scalar Higgs boson of
the MSSM behaves like the Higgs boson of the standard model and the other Higgs
bosons are all as heavy as the SUSY breaking scale. The strength of the
first-order EWPT is found to be dependent on the mass of the lightest neutral
Higgs boson and the mixing effects of the vector-like scalar quarks.
