The calibration of universal testing machines using calibrated force transducers (load cells) is very important for industry and research institutions in any country in the world. Some factors may affect the calibration results considerably. This study concerns the effect of some important factors related to the force measurements. The calibration of material testing machines does not only contain the calibration of the test forces, which is investigated in this paper, but also the calibration of other important quantities. This study deals with the effect of the curve fitting equation of the load cell used in calibration, the permissible error of the machine, the rotation of the load cell, the repeatability and reversibility effects. This work reveals that if the previous factors are not taken into consideration, erratic calibration results will be obtained. The experimental results of this study show that the calibration error may reach ± 0.4% due to the linear approximation of the load cell curve fitting. The results show also that the error may exceed ± 2.25% and ± 2.5% for each of the repeatability and the reversibility effects respectively.