The paper deals with determination of uncertainty of direct shear test results of soils. The uncertainties were determined for the direct shear apparatus of the Department of Geotechnics, Faculty of Civil Engineering, University of Zilina, Slovakia. Based on the calibration protocols of normal and shear force transducers and taking into account also method of evaluation of shear strength parameters by Slovak Technical Standard STN 72 1030, the standard uncertainty of the angle of internal friction and cohesion of the test example, introduced in the Standard, were determined. The results show, that standard uncertainty of the angle of internal friction was 0.33 degrees (1.31% of 24.55 degrees) and standard uncertainty of the cohesion was 1.26 kPa (5.69 % of 22.11 kPa). In the paper we will also show that for soils with extreme values of angle of internal friction up to about 70 degrees and cohesions up to about 200 kPa, standard uncertainty of the angle of internal friction will be 1.91 degrees and 6.99 kPa. As it will be seen, the percentage expression of standard uncertainty can lead sometime to misunderstanding. Since soils are very diverse, values of angle of internal friction and cohesion change in quite large extent and they are two "bonded" parameters, we would like to note, that concerning uncertainty, it is necessary to determine always concrete values of uncertainty of the angle of internal friction and cohesion for concrete soil.
