  We have studied equilibration with respect to isospin degree of freedom in
four 96 mass systems $^{96}Ru+^{96}Ru$, $^{96}Ru+^{96}Zr$,
  $^{96}Zr+^{96}Ru$, $^{96}Zr+^{96}Zr$ at 100 AMeV and 400 AMeV with isospin
dependent QMD.
  We propose that the neutron-proton differential rapidity distribution is a
sensitive probe to the degree of equilibration with respect to the isospin
degree of freedom. By analyzing the average $N/Z$ ratio of emitted nucleons,
light charged particles (LCP) and intermediate mass fragments (IMF), it is
found that there exist memory effect in multifragmentation process. The average
$N/Z$ ratio of IMF reduces largely as beam energy increases from 100 AMeV to
400 AMeV which may result from the change of the behavior of the isotope
distribution of IMF charges. The isotope distribution of IMF charges does also
show certain memory effect at 100 AMeV case but not at 400 AMeV case.
