DFT calculations demonstrated that the parent cage of both Lu3N@C-88 and Gd3N@C-88 is D-2 symmetrical C-88:81738 and their structures and stability are mainly determined by electron transfer interaction between the encaged cluster and the parent cage. However, for La3N@C-88, the parent cage is C-s symmetric C-88:81734, La3N cluster are pyramidal in most screened cages, and the structure and stability of La3N@C-88 is determined by both electron transfer interaction and size effect. The calculations can rationalize the recent experimental observations and clearly demonstrate that the size of encaged cluster play a substantial role in determining the structures and stability of M3N@C-88. (C) 2011 Elsevier B. V. All rights reserved.