  In this paper, an improved quasi-particle model is presented. Unlike the
previous approach of establishing quasi-particle model, we introduce a
classical background field (it is allowed to depend on the temperature) to deal
with the infinity of thermal vacuum energy which exists in previous
quasi-particle models. After taking into account the effect of this classical
background field, the partition function of quasi-particle system can be made
well-defined. Based on this and following the standard ensemble theory, we
construct a thermodynamically consistent quasi-particle model without the need
of any reformulation of statistical mechanics or thermodynamical consistency
relation. As an application of our model, we employ it to the case of (2+1)
flavor QGP at zero chemical potential and finite temperature and obtain a good
fit to the recent lattice simulation results of S. Borsanyi $et$ $al$. A
comparison of the result of our model with early calculations using other
models is also presented. It is shown that our method is general and can be
generalized to the case where the effective mass depends not only on the
temperature but also on the chemical potential.

High Energy Physics - Phenomenology;Nuclear Theory