This study deals with the enhancement of the thermal behavior of thermostable thermoplactics dedicated to high performance composites. In this aim, a semi-crystalline PEKK (Tg =159°C, Tm=360°C) and an amorphous PEI (Tg=220°C) have been blended at different ratios by using a high temperature co-rotative twin screw extruder. Blends have been injected using an injection-molding machine with a “cold” mold. We have performed on the injected samples, differential scanning calorimetry (DSC) and thermomechanical analysis which show the increase of the glass transition temperature (Tg). Due to the high cooling rate of the injection process, the injected samples are quenched and are consequently in the amorphous state. This opportunity allows us to measure their properties in this state considered as a reference. Then, samples are crystallized by annealing way at 250°C during 2 hours. As expected, in the amorphous state the Tg follow the Fox law, but a deviation is observed for the crystallized ones. In our talk, we will propose to analyze this deviation considering that the crystallinity level does not affect the Tg. On the contrary, crystallinity affects the composition of the amorphous phase. On this base, we will show that it is possible to describe quantitatively the evolution of the Tg for all blends and will propose a corrected Fox law to predict the Tg whatever the composition.