  We investigate the interaction effect between atoms and the finite size
effect of a Bose-Einstein gas at finite temperature. Using a mean field
approach, we derive the thermodynamic potential on finite systems and obtain
the condensate fraction, the chemical potential and the internal energy with
the interaction and the finite size effects taken into account. In this
approach, we can treat thermodynamic quantities as those of the ideal Bose gas.
It is found that the interaction effect shifts the transition temperature by a
few percent toward the lower temperature. Above the transition temperature,
these quantities agree with the noninteracting Bose gas.
