absolutely stable. An important issue is that whether the central density could achieve the value for phase transition. Unfortunately, this question is not easy to answer due to a large uncertainty of the MIT bag constant $B$. Schertler et al.[@sgt97] had included medium effects (by effective quark masses), the influence of which can’t be simulated by taking into account the coupling constant ($\alpha_{\rm s}$) or $B$ or strange quark mass ($m_{\rm s}$), in the study of SQM properties and found that medium effects reduce significantly the SQM binding energy (SQM could not be absolutely stable when $\alpha_{\rm s}>1$), but can hardly change the mass and radius of strange stars if they exist. However, a recent investigation[@bl99] shows that the phase transition from nuclear matter to quark matter can occur in proto-neutron stars as long as the bag constant $B\leq$ 126 MeV fm$^{-3}$. Various estimates of the bag constant[@bag_B] indicate that the preferred value of $B$ lies in the range of $60 {\rm MeV ~fm^{-3}}\leq B \leq 110 {\rm MeV
~fm^{-3}}$ (see, e.g., a review by Drago[@drago98]), which means that a phase transition is likely to happen. Furthermore, Drago & Tambini[@dt99] show that phase transition can occur at densities slightly larger 