Indentation size effect (ISE), whereby the strength of materials is observed to increase significantly with decreasing indentation depths has been reported in several experimental studies. In the present study, a series of nanoindentation experiments with maximum indentation depths varying from 400 to 3400 nm are carefully designed and implemented to study the indentation size effect on copper and Al7075. 3-D finite element analyses incorporating the conventional mechanism-based strain gradient (CMSG) plasticity theory which requires only C-0 solid elements are performed to simulate the indentation size effect. In order to circumvent the ambiguity associated with various methods to evaluate indentation hardness, the indentation size effect is investigated at the more fundamental level by considering changes to the load-displacement curve. The finite element results incorporating the CMSG plasticity theory are found to be in good agreement with experimental results at all levels of indentation depth conducted in the present study. (c) 2006 Elsevier B.V. All rights reserved.