  We analyze recent results of SU(3) lattice QCD calculations with a
phenomenological parametrization for the quark-gluon plasma equation of state
based on a quasi-particle picture with massive quarks and gluons. At high
temperature we obtain a good fit to the lattice data using perturbative thermal
quark and gluon masses from an improved HTL scheme. At temperatures close to
the confinement phase transition the fitted masses increase above the
perturbative value, and a non-zero (but small) bag constant is required to fit
the lattice data.
