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In this article,we consider the Cauchy problems for the modified Kawahara equation (6)tu + μ(6)x(u3) + α(6)5xu + β(6)3xu + γ(6)xu =0and the Kaup-Kupershmidt equation (6)tu + μu(6)2xu + α(6)5xu + β(6)3xu + γ(6)xu =0.Using the general well-posedness principle introduced by I.Bejenaru and T.Tao,we prove that the modified Kawahara equation is ill-posed for the initial data in Hs(R) with s ＜ -1/4 and that the Kaup-Kupershmidt equation is ill-posed for the initial data in Hs(R) with s＜0.