Abstract: Blends (LLDPE/PS) of linear low density polyethylene(LLDPE) with polystyrene(PS) were prepared by a reactive extrusion method. In order to increase the compatibility of two blending components, a kind of Lewis acid catalyst, AlCl₃, was adopted to initiate the Friedel-Crafts alkylation reaction between blending components. Due to the fact that in-situ generated LLDPE-g-PS copolymer could act as a compatibilizer in the relative blending system, the mechanical properties of LLDPE/PS system were greatly improved. For example, after compatibilization the Izod impact strength of a LLDPE/PS blend(80/20, by mass) was increased from 88.5 J/m to 401.6 J/m and its elongation-at-break increased from 370% to 790%. The Raman spectra of the LLDPE/PS blends extracted with tetrahydrofuran verified that LLDPE segments were grafted to the para-position of benzene rings of PS, which suggested that Friedel-Crafts alkylation did happen during reactive extrusion blending. SEM micrographs showed that the size of domains was decreased from 4~5 μm to less than 1 μm depending upon the mas fraction of AlCl₃ added. The crystallization behavior of LLDPE/PS blend was investigated by DSC. The fractionated crystallization phenomena were noticed, which could be mainly attributed to the confined crystallization rather than the reduction of the size of LLDPE domains.