Rheological properties of two poly(ester imide)s with a similar chemical structure are studied in this paper. Both polymers contain the same mesogenic unit and flexible spacer : the only difference is the orientation of the connecting ester groups. The liquid crystalline nature of one of the poly(ester imide)s in the range 170-235 degrees C gives rise to typical peculiar theological features of nematic polymer such as two shear-thinning regions in the viscosity curves, partial derivative eta/partial derivative T>0 when isotropization takes place and orientation during the injection process.