The energy band gap of a series of different sizes of CdSe quantum rods have been determined by STM technique at room temperature. The results confirm that the band gap of CdSe quantum rods (QRs) depends mainly on the width (the dimension of the electron confinement) and only slightly on the length as shown previously in the literatures. The experimental data is compared to that calculated using two theoretical models, the effective mass approximation (EMA) and the semi-empirical pseudopotential method (SEPM). The theoretical values for the energy band gap at varying radius are in agreement with the experimental results within 0.08 eV. (c) 2008 Published by Elsevier B.V.