Solid-liquid separation is a vital step in drilling sludge disposal, and the filterability and settleability of drilling sludge are the main evaluating indicators for the separation process. The influence of Na+, K+, Mg2+, Ca2+, and Fe3+ on drilling sludge filterability and settleability was investigated in our research. The water content, filtration rate, supernatant volume and supernatant turbidity were measured to evaluate the filterability and settleability of drilling sludge. Meanwhile, the zeta potential, specific surface area of sludge flocs, particle size distribution and Fourier-transformed infrared spectra were employed to clarify the influencing mechanism. The experimental results showed that the filterability and settleability of drilling sludge were related to concentration and types of cations. Mg2+, Ca2+, and Fe3+ performed better than Na+, K+, and the cations with smaller hydrated radius got superior solid-liquid separation behavior at same valence. Finally, the spectra indicated that no chemical adsorption occurred between inorganic cations and drilling sludge flocs. The variation of surface charge and flocs growth after adding different inorganic cationswere the reasons for the changes of the filterability and settleability. (C) 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.