This paper reports the effect of sol size on nanofiltration performances of sol-gel derived microporous zirconia membranes. Microstructure, pure water flux, molecular weight cut-off (MWCO) and salt retention of zirconia membranes derived from zirconia sols with different sizes were characterized. Thermal evolution, phase composition, microstructure and chemical stability of unsupported zirconia membranes (powder) were determined by thermogravimetric and differential thermal analysis, X-ray diffraction, nitrogen adsorption-desorption and static solubility measurements. Results show that nanofiltration performance of zirconia membranes is highly dependent on sol size. The sol with an average size of 3.8 nm, which is smaller than the pore size of the gamma-Al2O3 support (pore size: 5-6 nm), forms a discontinuous zirconia separation layer because of excessive penetration of sol into the support. This zirconia membrane displays a MWCO value towards polyethylene glycol higher than 4000 Da. A smooth and defect-free zirconia membrane with a MWCO value of 1195 Da (pore size: 1.75 nm) and relative high retention rates towards MgCl2 (76%) and CaCl2 (64%) was successfully fabricated by dip-coating the sol with an appropriate size of 8.6 nm. Zirconia sol with an average size of 12 nm exhibits colloidal nature and forms a zirconia membrane with a MWCO value of 2332 Da (pore size: 2.47 nm). This promising microporous zirconia membrane presents sufficiently high chemical stability in a wide pH range of 1-12. (C) 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.