Electroless Ni-alloy films were prepared as an electric resistor material, and the effect of heating on their resistance properties is reported, along with an overview of electric resistor materials and their essential properties.Ni-alloy films having stable resistance to elevated temperature were obtained by codeposition of a high melting point metal-W, Mo etc.-into the electroless Ni-P alloy films. The mechanism of thermal stabilization is explained as follows. In the case of W codeposition, W atom inhibits the crystallization of the Ni-P amorphous structure. In the case of Mo codeposition, a mechanism is quite different from W codeposition. Mo atom makes two phases (crystalline and amorphous) in the as-deposited state and this condition could be kept by proceeding the Mo segregation process between two phases.A pulse heating method, which can provide practical and quick evaluation of the thermal stability of resistor materials, was also described.