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Effect of isospin dependence of nucleon-nucieon cross sectionσNNiso on the neutron-proton ratio of nucleon emissions (n/p)free as the increase of neutron-proton ratio of collision system (n/p)sys is investigated in heavy ion collisions. Where the isospin independent N-N cross sectionσ1noiso andσ2noiso are defined asσnn=σpp =σnp(σ1noiso) andσnn=σpp= (1/2)(σnp+σnn) (σ2noiso) respectively. Because the isospin dependent in-medium N-N cross sectionσNNiso,especially the neutron-proton to proton-proton (neutron-neutron) cross section ration