The main purpose of the research work in framework of this thesis was the development or modification of CEM ("concrete equivalent method") for the design of equivalent mortar composition from concrete composition. The optimal formulation method should be determined based on the flow table test and rheological characteristics obtained by ConTec Viscometer 5. Based on the composition of concretes designed and tested in the framework of other diploma works we designed five reference mortar mixtures, which were later modified. We wanted to find out whether the reference mortar mixture is the best mixture to estimate the rheological properties of concrete or one of modified mixtures could be a better choice. All mixtures were composed of water, aggregate, cement and additives. The type of water and aggregate were the same for all mortars and we used two types of cement. In four mixtures we substituted part of the cement with mineral additive, microsilica or slag. As chemical admixtures we selected superplasticizer and air entraining agent. We focused on the results of flow table test, which represented the basis for the estimation of rheological characteristics of the equivalent mortar mixtures; these were later compared with the results obtained on concretes and correlations were made. Finally, based on the similar flow values we also compared rheological parameters of mortars and concretes acquired with the rheometer ConTec Viscometer 5.