The subject of this paper is the effect of changing the roughness of the inflow surface of an orifice on its characteristics. A sloping segmental orifice was chosen as the reducer and its surface roughness was changed by lining its inflow surface with sandpaper of different roughness parameters. In such a system, the dependence of the flow stream on the differential pressure at the orifice was measured on a test stand in the range of Reynolds numbers 4110-17000, the flow coefficient C was calculated and flow characteristics were prepared. The study was carried out for a selected segmental orifice with flow coefficient C= 0.723 installed in a pipeline with diameter Dw = 50.35 mm. The roughness of the inflow surface was changed by sticking sandpaper with a gradation of P120 ÷ P1200 to its surface. The roughness parameter Ra was in the range Ra = 5.69 μm ÷ 25.85 μm and Rz = 43.84 μm ÷ 164.83 μm. The dependence of the variation of the coefficient C on the roughness parameter Ra is then presented and the relative errors of the flow stream measurements were calculated. The paper also presents distributions of the differential pressure measured at the orifice for different flow streams depending on the roughness parameter.