A novel poly(aryl ether), poly(phthalazinone ether ketone sulfone) (PPESK) copolymer, was synthesized. This high performance polymer was characterized by an extremely high glass transition temperature, extraordinary thermooxidative stability, but poor processability. The melt blends of PPESK and liquid crystal copolyester were prepared. The rheology, morphology and the thermal behaviour of the blends were investigated by means of capillary rheometry, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric and heat deformation analysis. SEM observations revealed that a two-phase microstructure of the blends was formed, and a fibril morphology is created in the PPESK/LCP blends with the exception of PPESK/5%LCP blend. The thermogravimetric and heat deformation analysis indicated that the addition of LCP slightly affected these thermal properties of PPESK/LCP blends. The relationship of morphologies and rheological properties was correlated to explain the experimental results. The possible use of LCP as a processing aid for improving the processability of the novel PPESK was studied and discussed.