The present work reports an investigation on the rheological behaviour of fresh cement pastes. Three types of Portland cement were used. The water/cement ratio was varied in the range 0.35 ÷ 0.40. The rheological tests were performed using the coaxial-cylinder viscometer Rotovisko-Haake RV 11. The material to be tested was subjected immediately after mixing to a constant shear rate until a steady value of shear stress (equilibrium value) was attained. All the pastes tested exhibited a flow behaviour of the partially thixotropic type. A rheological model consisting of a sole constitutive equation\(\tau = \tau (\dot \gamma ,t)\) was defined according to the Cheng and Evans approach. The parameters of the constitutive equation were correlated with the cement specific surface and the water/cement ratio.