Metallocene linear low density polyethylene (mLLDPE) crystallization under shear flow at different controlled shear rates was investigated experimentally between its quiescent crystallization temperature and its melting point temperature (T). The evolution of the material optical properties, including turbidity, birefringence and dichroism, was monitored following a temperature jump from a temperature much higher than T. to a fixed crystallization temperature (T,). These properties are discussed in terms of the evolution of the polymer semi-crystalline microstructure. In light of the optical properties evolution, the crystallization process can be split into three stages (i) incubation phase in which small (compared to the light wavelength) crystalline nuclei spread over the medium, (ii) isotropic crystallitc growth phase and (iii) anisotropic crystallite growth phase. The optical properties evolution due the development of the crystallization is compared to that of the stress under the same thermo-mechanical conditions. It is observed that the optical properties are more sensitive than the stress to follow the crystalline development, in particular during the early stage of the process. (c) 2006 Elsevier Ltd. All rights reserved.