Wastewater treatment of machine-building enterprises contaminated with oils, petroleum products and mechanical impurities is an urgent task. A promising direction in the field of treatment of such wastewater is the use of pressure hydrocyclones and hydrocyclone installations. The article presents the results of research on the treatment of oil-containing wastewater of machine-building enterprises in pressure of hydrocyclones, conducted on an experimental hydrocyclone installation, which includes the pressure hydrocyclone under test, a sedative tank, and tanks for receiving water from the upper and lower drains of the hydrocyclone. The studies were carried out in two stages: the first stage was used to determine the diameter of the pressure of hydrocyclone suitable for the treatment of oily wastewater, and the second stage was used to study its geometric characteristics. At the first stage, seven modifications of pressure hydrocyclones with a diameter of 40 to 100 mm with different diameters of the upper and lower drain pipes were tested. A 75 mm diameter hydrocyclone is recommended for the treatment of oily wastewater. At the second stage, nine pressure hydrocyclones with a diameter of 75 mm with different diameters of the upper and lower drain pipes were tested. As a result of the experiments, the design parameters of the hydrocyclone that showed the best results were determined. The results of the research were used in the design of industrial hydrocyclone installations.