  The fission processes of thermal excited nuclei are conventionally studied by
statistical models which rely on inputs of phenomenological level densities and
potential barriers. Therefore the microscopic descriptions of spontaneous
fission and induced fission are very desirable for a unified understanding of
various fission processes.
  We propose to study the fission rates, at both low and high temperatures,
with microscopically calculated temperature-dependent fission barriers and
collective mass parameters.
  The fission barriers are calculated by the finite-temperature
Skyrme-Hartree-Fock+BCS method. The mass parameters are calculated by the
temperature-dependent cranking approximation. The thermal fission rates can be
obtained by the imaginary free energy approach at all temperatures, in which
fission barriers are naturally temperature dependent. The fission at low
temperatures can be described mainly as a barrier-tunneling process. While the
fission at high temperatures has to incorporate the reflection above barriers.
Our results of spontaneous fission rates reasonably agree with other studies
and experiments. The temperature dependencies of fission barrier heights and
curvatures have been discussed. The temperature dependent behaviors of mass
parameters have also been discussed. The thermal fission rates from low to high
temperatures with a smooth connection have been given by different approaches.
\item[Conclusions] Since the temperature dependencies of fission barrier
heights and curvatures, and the mass parameters can vary rapidly for different
nuclei, the microscopic descriptions of thermal fission rates are very
valuable. Our studies without free parameters provide a consistent picture to
study various fissions such as that in fast-neutron reactors, astrophysical
environments and fusion reactions for superheavy nuclei.
