The isospin effects of Coulomb interaction on the momentum dissipation in the heavy ion collissions at an intermediate energy has been studied carefully within the isospin dependent quantum molecular dynamics model. The results show that Co ulomb interaction induces the reduction of momentum dissipation because it leads the shrinking of the mechanical and chemical instability domains, and the degre e of reduction depends sensitively on the form and strength of symmetry potentia l. However, the property that the momentum dissipation depends sensitively on th e isospin dependences of in-medium nucleon_nucleon cross section and insensitiv ely on the symmetry potential, is true for the cases with and without the Coulom b interaction.