In this paper we study a rough order linear equation with periodic slowly timevaring coefficients x4+a1(t)+a2(t)+a3(t)+a4(t)x=e(t) (1)where ai(t)(i=1
2
3
4)
e(t) are all continuous
periodic differentiable function with period (2π/ω). By using [1]′s method we obtain a theorem on existence uniqueness and stability of periodic sollution for (1).