This master thesis is going to study and investigate the thermal treatment’s effects on the high Mn-TWIP steels. In order to get these aims some factors can be studied, such microstructures and mechanical properties of high Mn-TWIP steels. The objective of this study are as follows:  Study the microstructure evolution of high Mn-TWIP steels thermally treated (i.e. grain size, phase transformation, etc).  Study the mechanical properties (hardness, tensile properties and high cycle fatigue) of high Mn-TWIP steels with different microstructures.  Thermal fatigue evolution (from 0 up to 75 cycles) at 500°C in order to determine the microstructure evolution and their mechanical properties (harness and tensile properties). In present project, in order to study the microstructure the field emission scanning electron microscopy (FESEM) used. For testing the mechanical properties, Vickers hardness method applied for hardness test, tension condition for tensile test and high cycle fatigue has been used for fatigue properties. Microstructural study indicates that thermal treatment can change the microstructure of the samples, it observed that the grain size changed in various condition. Martensitic and pearlitic transformation occurred under thermal treatments and thermal fatigue conditions respectively. In samples which were thermally fatigued, the concentration of the pearlitic phase increased by increasing the number of cycles. Thermal treatments can have some effects on mechanical properties. Hardness of the materials decrease by increasing the temperature in thermal treatments while thermal fatigue increases the hardness of materials by increasing the number of cycles. Samples which were thermally treated and thermally fatigued showed more brittle behavior compared with the samples which were not thermally treated. It have seen that the voids, TiN and pearlitic phase are the main reasons of fracture under tensile tests. Finally it observed that the life time of the materials can be affected by annealing the samples at 1000°C and normalizing this sample in air.