Development of X-ray computed tomography (XCT) in recent years has led to its increasing use in industry for dimensional metrology and non-destructive testing. However, measurement uncertainty of the XCT has not been assured so far and one of the key influencing factors that affects measurement uncertainty is the dimensional stability of the XCT scanner. X-ray source used in XCT scanner generates heat which makes the whole X-ray source assembly susceptible to thermal expansion which in turn increases measurement uncertainty. In this thesis, the foundations for research of thermal expansion effects on measurement uncertainty are set. These include geometrical calculations for determining the influence of X-ray source expansion, and manipulator positioning errors on geometrical magnification, which translates to error in measurement result. Additionally, the XCT scanner located in Laboratory for precision length measurements at Faculty of mechanical engineering and naval architecture has been 3D optically scanned and measured. The 3D scanning results have been used for reconstruction of simplified CAD models of XCT’s functional components (X-ray source, manipulator and detector). Reconstructed CAD data can be used in further analysis, e.g. determination of displacement by using finite element method. Finally, an experimental investigation of dimensional stability of XCT has been conducted by scanning on two different X-ray source power levels. During the scanning, the temperature has been measured on various locations inside the XCT. The results show that the X-ray source temperature varies significantly, causing thermal expansion of the source which changes geometrical magnification and shifts the projection. Approximate values of X-ray source expansion and manipulator positioning errors have been calculated by using the presented geometrical calculations.