The $[Ni_{80}Fe_{20}/Cu/Co/Cu]$ multilayers were grown by evaporation technique, and according to magnetic exchange coupling relation, magnetoresistance ratio and magnetization curve were studied by Co inserting $Ni_{80}Fe_{20}/Cu$ interface. Insertion of Co layer to the antiferromagnetically coupled system, i. e $t_{Cu} = 25;{AA}$, decrese the MR ratio contrary to previous reports. However the insertion to the ferromagneticalyl coupled $(t_{Cu} = 27;{AA})$ and the noncoupled $(t_{Cu} = 47;{AA})$ systems increase the ratio to 3.5 % and 6 % respectively. The results imply that the insertion change the magnetic exchange coupling state as well as the spin dependent scattering of conduction electrons. Besides, insertion of Co layer between Cu and $Ni_{80}Fe_{20}$ layer enhaces thermal stability to the 300 $^{circ}C$, which indicates that insertion of Co has a role of the effective diffusion barrier.