Average position of electrons along thickness direction in a Coulomb island in an n-channel Si single-electron transistor is estimated by analyzing the back-gate voltage dependence of peak voltage (defined as the gate voltage giving a drain current peak) as a function of peak number. It is found that the accuracy of estimated average position is better than 0.5 nm and that the average position fluctuates as the peak number increases. (C) 2011 Elsevier B. V. All rights reserved.