Additive manufacturing (AM) is a novel technology that enables rapid fabrication of physical models directly from 3D computer-aided design (CAD) data without any conventional tooling or programming requirement. Thermoplastic polymers are the most useful materials for the manufacturing of parts in the FFF process. In this process, extrusion of a semi-molten road through a nozzle is taken place to form each layer, the extruded road solidifies quickly due to the existence of temperature gradient between the surroundings and the extrusion temperature. Different key parameters affect the final products manufactured by this process. These parameters can be listed in three categories. Some of them are linked to the material, others are linked either to the characteristics of the process or to the specificity of the machine. They can influence the properties of the final part through their effect on various physical phenomena. The mentioned parameters affect the polymer temperature and its evolution. It is important to know the evolution of filaments temperature with time and recognize how it is affected by major process variables as mentioned. Due to the nature of the FFF process, it is important to measure the temperature profile and its evolution during the process by the means of local measurement methods. The idea of this work took place in 2018, by start reviewing literatures related to the FFF process. As mentioned, almost all studies and works either numerical or experimental approaches were based on global consideration. In the beginning, the work was concentrated on finding a method to be applied to the FFF process to proceed with the localized investigation. Afterward, the experiment was started to see the possibility of the work. As in the FFF process, there is a deposition of filaments, and each filament itself is heated by the deposition of newer filaments, there is almost a cyclic evolution of the temperature due to multi-layer d eposition and it means that each filament is re-heated consequently because of the deposition of a new filament. This is a critical issue in creating a filament bonding and diffusion of materials. To implement and measure this cyclic temperature, it is required to apply a measurement device in which to be capable of measuring the temperature of the polymer when leaving the nozzle. One can note that the mechanical properties of 3d-printed pieces are limited. In this work, we try to improve the mechanical properties by reinforcing the fibers such as glass fibers, carbon fibers, etc. At the same time by controlling the temperature evolution, we try to improve the adhesion between the layers to have the best structure. The used material as raw material was polyamide-6 (PA6). The main objective of this research is to study the rheological characteristics of materials during FDM/FFF to process optimization for mechanical characterization improvement of the fabricated parts. Therefore, the main objective is to take into account both the temperature and viscosity parameters, and to establish the Time-Temperature-Transformation diagram for process optimization. This helps to determine the processability area.