By using the exponential dichotomy and the averaging method,a perturbation theoryis established for the almost periodic solutions of an almost differential system.Suppose that the almost periodic differential system(dx)/(dt)=f(x,t)+ε2g(x,t,ε)(1)has an almost periodic solution x=x0(t,M)for ε=0,where M=(m1,…,mk)is theparameter vector.The author discusses the conditions under which(1)has an almostperiodic solution x=x(t,ε)such that x(t,ε)=x0(t,M)holds uniformly.The results obtained are quite complete.