In order to explore the optimal relationship between eccentricity and mud-cement density difference in cement displacement process, the flow characteristics for non-Newtonian fluids involved in the laminar displacement inside an eccentric annulus are analyzed with fluid mechanics theory. An optimal density difference criterion is established based on the Herschel-Bulkley fluid model, which can be used to theoretically determine the optimum density difference yielding an optimal cement displacement performance. Numerical simulations are performed with a CFD software to investigate the influence of mud cement density difference on displacement performance under different eccentricities in highly deviated wells. The results indicate that there is an optimal density difference which can provide the best displacement efficiency at given inclination and eccentricity under laminar flow conditions. The good agreement, between the optimal density differences calculated by the optimal density difference formula and CFD numerical simulations, confirms the validity of the optimal density difference criterion for determining the optimal density difference in highly deviated wells. The research results can provide a practical guide on how to design the cement slurry density and how to install centralizers at highly deviated wells under laminar flow conditions. (C) 2016 Elsevier B.V. All rights reserved.