The size-fractionation of multi-walled carbon nanotube (MWNT) in the dimethylformamide (DMF) solution of poly(vinylidene fluoride) (PVDF) has been achieved by controlled sonication-centrifugation cycles. The Z-scan experiments and fluence-dependent transmission measurements with nanosecond laser pulses of 532 nm wavelength show that the optical limiting behavior of MWNTs in PVDF/DMF solution is size-dependent. The tubes of large aspect ratio possess stronger limiting properties, The size-dependent effect is more obvious at low input fluence than at high input fluence. (C) 2002 Elsevier Science B.V. All rights reserved.