The principles for measuring the water content of oil products in pipelines are proposed. They are based on the interaction between electromagnetic waves and the watered oil flow when the waves pass through the flow and reflect from them. Theoretical analysis of characteristics of the waves interacting with the controlled medium that link the amplitudes of their electric fields with the water content and grade of the oil is given. The analytical relationship between the output parameter of the amplitude detector and the water content of the oil and its dielectric permittivity is shown. The experimental results of the laboratory tests of microwave sensors for water content measurement in the oil flow are demonstrated.