Blends of wholly aromatic new thermoplastic polyimide (N-TPI) and poly(ether sulfone) (PES) were prepared by melt-mixing and subsequent injection molding. Their mechanical properties, morphology, and crystallization behavior were investigated. A synergistic effect on the flexural properties was observed for the N-TPI/PES blends over the whole compositions. Differential scanning calorimetric analysis confirmed that the blend is an immiscible system and that the crystallinity of the N-TPI component is very low (0.4-1.4%) irrespective of the composition. Scanning electron microscopic analysis suggested that the improvement in flexural properties is likely due to the reinforcement of the PES matrix by the fibrous N-TPI phase for the N-TPI/PES (40/60, 20/80) blends. For N-TPI/PES (80/20, 60/40), the orientation of the matrix N-TPI rather than the fibrillation of the PES phase along the melt-flow direction may contribute to the synergism observed. Also, study of the isothermal crystallization behavior of the blends revealed that the addition of 10 wt % PES accelerates the crystallization of N-TPI and further addition causes the retardation of it.