MR Diffusion kurtosis imaging (DKI) was proposed recently to study the deviation of water diffusion from Gaussian distribution. Mean kurtosis (MK), directionally averaged kurtosis, has been shown to be useful in assessing pathophysilogical changes. However, MK is not sensitive to kurtosis change occurring along a specific direction. Therefore, orthogonal transformation of the 4th order kurtosis tensor was introduced in the current study to compute kurtoses along the 3 eigenvector directions of the 2nd order diffusion tensor. Such axial (K( parallel)) and radial (K perpendicular) kurtoses measured the kurtoses along the directions parallel and perpendicular, respectively, to the principal diffusion direction. DKI experiments were performed in normal adult and formalin-fixed rat brain, and developmental brains. The results showed that directional kurtosis analysis revealed different information for tissue characterization.