Abstract: Centrifuge is a common equipment for field drilling fluid cleaning and is a solids control equipment with steady solids control efficiency. Two models of centrifuge, LW355 and LW450, are presently commonly used in field operations. For high rate of treatment, model LW600 is also used. Whatever the model is, the separation efficiency and the solid composition of the down-flow of the centrifuge do not vary much. This is because for a fluid with certain density and viscosity, an ideal "cut point" exists for the centrifuge to deal with. The composition and properties of a drilling fluid is always changing and in maintenance in field operations, the ideal "cut point" is changing accordingly, but most centrifuges presently used do not have the ability to automatically regulate its "cut point" to the ideal value. Hence, the composition of the solids in both down-flow and overflow of the discharge should be analyzed continually to monitor the real-time treatment efficiency of the centrifuge and to adjust the separation efficiency to an ideal state as far as possible based on the solids composition analyses. Analyses on the composition of down-flow solids gathered from centrifuges used in oilfields in China showed that there is an obvious difference between the separation efficiencies for high density solids and low density solids. This is because when a centrifuge is in operation, change in the feed state of the centrifuge is continuously changing, resulting in random change in the "cut point" of the centrifuge. A systematic analysis on the solids composition of the discharge of the centrifuge is helpful to advancing the technology and improving the separation efficiency of centrifuge.