We introduce the isospin degree of freedom into the momentum-dependent interaction to obtain an isospin and momentum dependent interaction given in a form practically usable in isospin-dependent quantum molecular dynamics model.We investigate the isospin effect of momentum-dependent interaction on the observable in the intermediate-energy heavy ion collisions.It is found that momentum-dependent interactions with and without isospin dependence,lead to significant different predictions on the nuclear stopping(momentum dissipation) and isospin fractionation ratio.In particular,the momentum-dependent interaction with the isospin degree of freedom induces the reduction for the strong dependence of isospin fractiona tion ratio on the symmetric potential.However,it does not change obviously the a bove feature for the isospin fractionation ratio as the good probe of symmetric potential respectively.The isospin dependence of momentum-dependent interaction is thus important for studying accurately the equation of state of isospin asym metry nuclear matter.