  In this paper, we show that a generalized Sasakian space form of dimension
greater than three is either of constant sectional curvature; or a canal
hypersurface in Euclidean or Minkowski spaces; or locally a certain type of
twisted product of a real line and a flat almost Hermitian manifold; or locally
a wapred product of a real line and a generalized complex space form; or an
$\alpha$-Sasakian space form; or it is of five dimension and admits an
$\alpha$-Sasakian Einstein structure. In particular, a local classification for
generalized Sasakian space forms of dimension greater than five is obtained. A
local classification of Riemannian manifolds of quasi constant sectional
curvature of dimension greater than three is also given in this paper.
