In a previous article, we described the synthesis and magnetic characterization of superparamagnetic elongated assembly of iron oxide nanoparticles and block copolymers by electrostatic interactions. We demonstrated that those rod-like aggregates preserved the superparmagnetic behavior of the magnetic nanoparticles that are included in it. Here we investigate the dynamic behavior of those rods using optical dynamic birefringence measurements on rods samples made from different lengths. Herein we confirme the superparamagnetic behavior of the magnetic rods and emphasize a new method for the determination the ensemble averaged length of the rods by a macroscopic optical method. (C) 2010 Published by Elsevier Ltd