Aliphatic polyamide (PA 6 or PA 66), polysulfone (PSU) and polysulfone copolymerized with 4,4＇-bis(4-hydroxyphenyl)pentanoic acid bearing carboxylic acid functions (PSU copolymers) were melt blended in the MINI-MAX moulder. Morphology of the blends was characterized by image analysis on SEM pictures and light scattering. The particle size depends on the viscosity ratio and on the concentration of COOH functional units in the PSU. The carboxylic acid function indeed reacts with amino end groups of the polyamide, leading to the formation of a grafted copolymer concentrated at the interface. In this way, interfacial tension is reduced and interfacial adhesion enhanced. The mechanical properties of PA/PSU blends are also improved by the formation of this grafted copolymer. However, considering mechanical properties, particle size appears as the dominant parameter. Mechanism of deformation and fracture of these blends are also discussed.