Abstract: Research work in this paper is done about drag reduction by aeration from the view of basic equations of fluid mechanics. By analyzing phases velocity profiles and resistance law resulted from gas slug in the pipe center, the condition that the secondary flow occurs, phases velocity profiles, formula of drag reduction and a curve of drag reduction have been obtained. The curve shows that the drag reduction law is affected by proportion of gas, and the drag reduction rarely happens in slug flow. While gas proportion is in a certain scope, the drag resistance is aggrandized greatly. Therefore some measures should be taken to avoid slug pattern while drag reduction by aeration is applied.