Poly(phthalazinone ether sulfone ketone) (PPESK) was melt blended with bisphenol-A polysulfone oligomer (O-PSF)to produce a thermoplastic polymer blends. The miscibility, thermal stability, theological and mechanical properties of the blends were investigated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC),thermogravimetric analysis (TGA) and capillary rheometry. The blends showed single Tg over the composition range and possess homogeneous microstructure. The addition of O-PSF slightly affected the thermal properties of the blends. PSF oligomer, as a processing aid, could markedly improve the processability of the PPESK. In addition, the mechanical properties of the blends were increased, to some degree, by adding O-PSF.