  The degree of isospin fractionation is measured by $(N/Z)_{n}$ /
$(N/Z)_{N_{imf}}$, where $(N/Z)_{n}$ and $(N/Z)_{N_{imf}}$ are the saturated
neutron-proton ratio of nucleon emissions (gas phase) and that of fragment
emissions (liquid phase) in heavy ion collision at intermediate energy . The
calculated results by using the isospin-dependent quantum molecular dynamics
model show that the degree of isospin fractionation is sensitive to the
neutron-proton ratio of colliding system but insensitive to the difference
between the neutron-proton ratio of target and that of projectile. In
particular, the degree of isospin fractionation sensitively depends on the
symmetry potential. However its dependences on the isospin dependent in-medium
nucleon-nucleon cross section and momentum dependent interaction are rather
weak.
  The nucleon emission (gas phase) mainly determines the dynamical behavior of
the degree of isospin fractionation in the isospin fractionation process,
compared to the effect of fragment emission. In this case, we propose that
$(N/Z)_{n}$ / $(N/Z)_{N_{imf}}$ or $(N/Z)_{n}$ can be directly compared with
the experimental data so that the information about symmetry potential can be
obtained
