Polymer blends based on poly(ethylene 2,6-naphthalate) (PEN) and poly(ethylene terephthalate) (PET) reinforced with a thermotropic liquid crystal polymer (TLCP) were prepared using a melt blending process. Polymer blends consisting of conventional cheap polyester with a small quantity of expensive TLCP are of interest from an economic point of view. The shear viscosity of the TLCP and polyester blends decreased with increasing shear rate and depended on TLCP content. The lower values of the structural viscosity index for the TLCP and polyester blends were attributed to the formation of fibrillar TLCP structures having elongated fibrils in the polyester matrix. The TLCP/PEN blends exhibited long TLCP fibrils that had smaller average diameters and narrower distributions of the diameter compared with those of the TLCP/PET blends. The higher shear force and lower viscosity ratio observed may favor the in situ TLCP fibril formation in the polyester matrix. The viscosity ratio was the most crucial factor in controlling the morphology of the TLCP phase in the TLCP and polyester blends. (c) 2005 Wiley Periodicals, Inc.