Nonlinear optical properties of hydroxyl groups modified multi-walled carbon nanotubes (MWNTs-OH) with diameter of 20-30 nm (MWNTs2030-OH) dispersed in water, N,N-dimethylformamide (DMF) and chloroform were studied by using Open-aperture Z-scan technique with nanosecond pulse at 532 nm. The influences of solvent, laser pulse-width, on-axis fluence at focus and concentration on nonlinear optical properties of MWNTs2030-OH were analyzed in detail to obtain the optimal nonlinear optical response for applications in optical limiters. Nonlinear optical properties of MWNTs-OH with different diameter in DMF were also studied and compared with multi-walled carbon nanotubes in DMF and C-60 in toluene. (C) 2010 Elsevier B. V. All rights reserved.