  We investigate a novel mechanism of bulk acceleration for relativistic jets
of pure electron-positron pair plasma.We assumed that relativistically hot
plasma is optically thick for Compton scattering and then pair plasma coupled
dynamically to photons may establish quasi-Wien equilibria. We name such an
electron-positron pair plasma the "Wien-fireball". Assuming there is a
Wien-fireball, it is easy to form highly relativistic jets not only because
coupled photons do not cause radiation drag effect, but also because coupled
photons prevent catastrophic annihilation of pair plasma. It is found that the
Wien-fireball provides a very effective mechanism to form high power jets. And
the radiation of the Wien-fireball in blazars should be observed as MeV peaked
emission at infinity with an order of kinetic power of jets.
