Abstract: Ageing has significant effect on soil liquefaction resistance, and the quantitative characterization of ageing effect is one of the key research themes in assessment of soil liquefaction. An ageing index (AI) characterizing soil ageing effect is proposed based on small strain shear modulus (G_max) and cone penetration resistance of CPT testing (q_c), with further normalization of soil density and stress state. This index is expressed by a function of the ratio between the fabric parameter of small strain shear modulus and that of large strain penetration resistance. The analysis of laboratory test data shows that the reconstituted sand has a stable reference AI value, while the field data reveal that AI increases with the increase of geologic age of the soil deposit. AI is an apparent parameter as it can characterize the extent of soil ageing without the variable of time, which provides a sound basis for further researches on the effect of soil ageing on liquefaction resistance.