The basic structural unit of layered double hydroxides (LDHs) layers,MO6,has been investigated by density functional theory (DFT) calculations at (U)B3LYP/LANL2DZ/6-31G(d) level modeled as [M(OH2)6]n+ (M =metal cation,n =2,3).Geometric optimization calculation results quantitatively revealed that the construction of LDHs layers is more closely related to the distortion angle of MO6 (M=introduced cation) octahedral unit than the ion size.The metal cations which can form the canonical basic octahedral unit with the distortion angle (θ) smaller than 1 ° are easily incorporated into LDHs layers to form stable structures,while those which form octahedral unit structure in heavy distortion with θ larger than 10° are difficult to be introduced into LDHs layers.This is in agreement with the experimental findings and appears to be a more rational criterion than the empirical rule based on ion-size for the design and preparation of LDHs materials.