We have developed a nano-beam SIMS apparatus with the goal of performing nano-dimensional elemental analysis. For any type of analysis with high spatial resolution, it is important to estimate the lateral resolution or beam size. First, some of the important principles of the nano-beam SIMS apparatus are reviewed. Second, we estimated the beam size by using the knife-edge method and then we construct a model for the knife-edge profile consisting of three factors. Finally, we discuss the effective beam size, the low intensity tail outside of the beam center and the spread of the collision cascade. (C) 2004 Elsevier B.V. All rights reserved.