The degree of isospin fractionation is measured by the ratio of saturated neutron-proton:i.e. the ratio of gas phase (nucleon emission) to that of liquid phase (fragment emission) in heavy ion collisions. We have studied the dependence of the degree of isospin fractionation on the neutron-proton ratio in the colliding system by using isospin-dependent quantum molecular dynamical model. The calculated results show that the degree of isospin fractionation depends sensitively on the symmetry potential and weakly on the isospin effect of nucleon-nucleon cross section. In particular, the degree of isospin fractionation increases with increasing neutron-proton ratio in the colliding system for the neutron-rich system, in this process the neutron-rich gas phase and neutron-poor liquid phase are produced. The degree of isospin fractionation is very sensitive to the degree of symmetry potential. On the contrary, for the neutron-poor system the neutron-poor gas phase and neutron-rich liquid phase are produced. In this case, the degree of isospin fractionation is not sensitive to the symmetry potential. We also find that the role of momentum dependent interaction in the isospin fractionation process is not obvious. We propose that our calculated results can compare directly with the experimental data to get the information about the symmetry potential in the intermediate energy heavy-ion collisions.