Residual stresses are extremely important in terms of the operating life of the machine part, since they are present in the material even when there is no external force and are the result of their manufacture processes such as casting, welding, forming, heat treatments, etc. Residual stresses can generally be divided as tensile or compressive. In our work we measured the residual stresses in friction stir welded aluminium alloy 7075. The samples were welded with a tool made of hot work tool steel and with parameters 950 and 750 rpm, and welding speeds 73, 93 and 116 mm / min. The residual stresses were measured using the X-ray diffraction methodology across the whole weld and heat affected zone. They were measured in longitudinal and transversal direction of the weld face. The amount of residual stresses is mostly affected by the heat input. In comparison with other welding processes, friction stir welding has a smaller linear heat input, and lower residual stresses values can be expected. We found out, that due to friction stir welding in the weld area and in the HAZ, tensile residual stresses occur. Residual stresses occur as a result of heat and deformation, during friction stir welding. Due to heat and deformation, there are changes in mechanical properties. Welds with higher tensile residual stresses have higher hardness and lower tensile strength.