Laboratory mechanical tests were carried out to study the intrinsic variability of rock salt specimens obtained from the Middle and Lower members of the Maha Sarakham formation in the Khorat basin. Prior to the mechanical tests, the types and amount of inclusions were identified by visual examination and after testing by X-ray diffraction and dissolution methods. The uniaxial compressive strength of the specimens linearly increases from 27 MPa to about 40 MPa as the anhydrite content (by weight) increases from 0% (pure halite) to 100% (pure anhydrite). The combined stiffness of the salt and the anhydrite causes an increase of specimen elasticity from 22 GPa (pure salt) to as high as 36 GPa (pure anhydrite). Tensile strengths increase with increasing anhydrite content, particularly when the content is above 60% by weight. Below this limit the anhydrite has an insignificant impact on the specimen tensile strength. The tensile strength of salt crystals can be as high as 2 MPa, whereas that of the inter-crystalline boundaries is 1 MPa. The visco-plasticity increases exponentially with crystal size, as dislocation glide mechanisms become predominant for the specimens comprising large crystals. Salt specimens with fine crystals deform by dislocation climb mechanisms, and hence reduce the specimen's visco-plasticity. The true understanding of the impact of the inclusions on the mechanical properties of the salt is important for the design and stability analysis of underground mines and storage caverns.