The effects of annealing in magnetic field after deposition on electromagnetic properties of $Ni_{81}Fe_{19}$ thin($400;{AA}$) films prepared by RF-magnetron sputtering were investigated in terms of microstructure and surface morphology. The coercivity of the films was decreased below $300^{circ}C$ due to stress relief and recrystallization, while increased at $400^{circ}C$ due to grain growth and increasing the surface roughness. And then, $4{pi}M_{s}$, was almost independent of annealing temperatures. Increasing the annealing temperature. the electrical resistivity of films was decreased from $37;{mu}{Omega}cm$ to $24;{mu}{Omega}cm$, the magnetoresistance was nearly a constant of about $0.6;{mu}{Omega}cm$, and the MR ratio was increased from 1.5 % to 3.1 %. Therefore, It was shown that increasing the magnetoresistive ratio was mainly affected by decreasing the electrical resistivity. Considering the practical application of the films for magnetoresistive heads, optimal annealing conditions was obtained after one hour annealing at $300^{circ}C$ in 400 Oe unidirectional magnetic field.