  The phase transition for SU(3) gauge field (without quarks) is considered. It
is shown that the phase transition is due to the fact that at high temperatures
the partition function should be calculated as for a gas of gluons, whereas at
low temperatures as the sum over energy levels of correlated quantum states of
SU(3) gauge field. A correlated quantum state for strongly interacting fields
is defined as a nonperturbative quantum state of strongly interacting fields.
The energy spectrum of these quantum states are discrete one. A lower bound of
the phase transition temperature by comparing of the average energy for the
perturbative and nonperturbative regimes is estimated (for glueball being in
thermal equilibrium with the thermostat). It is shown that this quantity is
associated with a mass gap. In a scalar model of glueball its energy is
calculated. It is shown that this energy is the mass gap. If we set the
glueball mass $ \approx 1.5 \cdot 10^3$ Mev then it is found that the
corresponding value of coupling constant lies in the nonperturbative region.
