We study the isospin effects of the mean field and two-body collision on the nucleon emissions at the intermediate energy heavy-ion collisions by using an isospin-dependent transport theory. The calculated results show that the nucleon emission number Nn depends sensitively on the isospin effect of nucleon-nucleon cross section and weakly on the isospin-dependent mean field for neutron-poor system in higher beam energy region. In particular,the correlation between the medium correction of two-body collision and the momentum-dependent interaction enhances the dependence of nucleon emission number Nn on the isospin effect of nucleon-nucleon cross section.On the contrary, the ratio of the neutron-proton ratio of the gas phase to the neutron-proton ratio of the liquid phase, i.e., the degree of isospin fractionation b / b depends sensitively on the isospindependent mean field and weakly on the isospin effect of two-body collision for neutron-rich system in the lower beam energy region. In this case, Nn and b/b are the probes for extracting the information about the isospin-dependent nucleon-nucleon cross section in the medium and the isospin-dependent mean field,respectively.