High manganese steels with low stacking fault energy due to twinning induced plasticity (TWIP) are capable of absorbing high energy at high strains such as accidents. This type of steel has been considered by the automotive industry and researchers because of this feature. Application of this steels in the car body along with other high ductility steels, such as Interstitial free steels (IF), has made the weldability of this steel very important. In this study, high energy beam welding (laser) was used for similar TWIP / TWIP and dissimilar TWIP / IF joint due to low input temperature, low heat affected zone and high speed compared to other common welding methods. For this purpose, primary TWIP steel (thickness 3 mm) was rolled up to 70% thickness reduction (1 mm) for TWIP / TWIP joint and up to 80% thickness reduction (0.7 mm) for TWIP / IF joint. Then they were annealed at 800 ° C for 10 and 30 minutes to obtain the desired structure. First, the effect of input heat (laser beam power) on growth morphology, solidification mode and microstructural changes due to laser beam welding process was investigated. Then the parameters affecting the laser welding process (beam power, welding speed and spot size) were identified and tests were designed and performed based on central composite design (CCD) in five levels (20 tests in total). Using the Response surface method (RSM), a mathematical model was proposed to predict the mechanical properties and chemical composition of the fusion zone. According to the proposed model, optimal conditions (power = 2595, speed = 0.54 m / min, spot size = 0.48 mm) were extracted and welding was performed based on these conditions. The results of actual test (final strength = 800 MPa) and model prediction (final strength = 839 MPa) were compared and validated, which confirmed the high ability of the model to predict properties. The results of response surface methodology analysis, showed that the welding speed and laser beam power have the maximum effect on the ultimate strength and the percentage of manganese in the melting zone. The microstructure, chemical composition and phases of the base metal and the fusion zone of the TWIP / TWIP and TWIP / IF joint were evaluated by optical microscopy, scanning electron microscopy (SEM), electron diffraction spectroscopy (EDS) and X-ray diffraction (XRD). In order to investigate the mechanical properties of the joint, the tensile test and microhardness (HV) test were used. EBSD analysis was used to investigate the crystallographic orientation relationship of different welding zones, distribution of special and twin boundaries and their effect on fracture resistant, texture transformations during deformation and the internal strain measurement. The grain size distribution diagram showed that the average grain size in the base metal is 3±0.4µm and in the fusion zone is 8±1µm, and some grains have grown up to 45µm , which has led to softening of the fusion zone. The decrease of the hardness from the mean of 315±4 HV in the base metal to 275±6 HV in the fusion zone, confirmed softening phenomena. Pole figures of the base metal and fusion zone at the fusion boundary region (epitaxial growth) indicates that the cube-cube orientation relationship ( 100 // 100 and {100} // {100}) along the boundary it is formed. The results of the optimal joint tensile test showed that the joint ultimate strength (805 MPa) is noticeable compared to the base metal ultimate strength (846 MPa). While the fracture occurred from the fusion zone, the reason for which, in addition to the decrease in hardness due to the coarseness of the grains, was attributed to the decrease of special and twin boundaries and the increase of random boundaries in the fusion zone. Investigation of texture evolution and fracture mechanism of the fusion zone after deformation showed that the weak cube-type texture in the fusion zone was removed during the deformation and the brass-type texture due to twin deformation intensified. Evaluation of the mechanical properties of the TWIP / IF dissimilar joint showed that in the tensile test, no fractures occurred in the IF-HAZ (fracture occurred from the IF-BM) due to the presence of very strong texture (111) or gamma fibers in the IF-HAZ region, which has a high resistance to deformation. Keywords : high manganese steels, Mathematical model, Response surface methodology, Orientation relationship, Texture, Special boundaries, misorientation angles.