The slow cylinder-to-sphere transition for cylindrical micelles from nanopore extrusion is quantitatively investigated by using transmission electron microscopy. The time-dependent length distribution and weight-average length of cylindrical micelles from experiment were compared with those calculated ones from computer simulation on the basis of end-scission, random-scission and Gaussian-scission models. The results reveal that the cylinder-to-sphere transition involves a combination of the Gaussian-scission model and the end-scission model and the scission rate constant is nearly a linear function of micelle length but slightly increases with time for a given length. As expected, such a thermal agitation induced transition with the spherical phase as its thermodynamically stable state is different previously observed in the shear-induced fragmentation of long cylindrical micelles made of amphiphilic block copolymers in selective solvent.