A stability analysis of the Bianchi type-I universe in pure gravity theory
is studied in detail, We first derive the nonredundant field equation of th
e system by introducing the generalized Bianchi type-I metric. This nonredu
ndant equation reduces to the Friedmann equation in the isotropic limit. It
is shown further that any unstable mode of the isotropic perturbation with
respect to a de Sitter background is also unstable with respect to anisotr
opic perturbations. The implications of the choice of physical theories are
discussed in detail in this paper.