  The collisions between linear polar molecules, trapped in a microwave field
with circular polarization, are theoretically analyzed. The microwave trap
suggested by DeMille \cite{DeMille} seems to be rather advantageous in
comparison with other traps. Here we have demonstrated that the microwave trap
can provide a successful evaporative cooling for polar molecules in a rather
broad range of frequencies of the AC-field. We suggested that not only ground
state polar molecules but also molecules in some other states can be safely
trapped.
  But the state in which molecules can be safely loaded and trapped depends on
the frequency of the AC-field.
