The dilatometer probe is used for in-situ measurements to determine deformation properties of rock massif in geotechnical investigations for underground and dam constructions. The common method and newly proposed approach for test evaluation are discussed. The effects of Poisson's ratio, incorporating the plastic zone development during the test as well as considerations of finite deformation (influence) zone are described. A parametric study on ranges of plastic and or deformation zone radius is presented and discussed.