This study focuses on the influence of molecular weight on the rheological, thermal, and mechanical behavior of poly(ether-ether-ketone) (PEEK), a semi-crystalline high-performance polymer The results show that the molecular weight of PEEK has significant influence on its rheological, thermal, and mechanical behavior It was found that PEEK has the unique characteristic of two shear-thinning regions The shear viscosity and the stress relaxation time of PEEK increase significantly as molecular weight increases In general, the Cox-Merz rule is valid for all grades of PEEK As molecular weight increases, the melting temperature of PEEK decreases slightly, but its isothermal and nonisothermal crystallization temperatures drop dramatically As molecular weight increases, the crystallinity, the crystallization rate, and the magnitude of crystallization activation energy decrease The crystallization kinetics study indicates that PEEK tends to form spherical crystalline structures, regardless of its molecular weight As molecular weight increases, the tensile strength at yield, the tensile modulus, and the flexural modulus of PEEK decrease slightly, whereas the tensile strength at break, the tensile strain at break, the modulus of toughness, and the impact strength of PEEK increase significantly POLYM ENG SCI, 51 94-102, (C) 2011 2010 Society of Plastics Engineers