Abstract: For the purpose of understanding the effect of aggregate size on the strength of filled soils,twelve types of soil samples were prepared for a series of unconfined compression and shear tests according to soil type and aggregate size.It was shown from the test results that(1) with average grain size of aggregates less than 3.5 mm,the cohesion of raw soil,reinforced soil with 0.4% fibre and treated soil with 8% lime decreased and the corresponding internal friction angles increased with increase of aggregate size;(2) with average grain size of aggregates greater than 3.5 mm,the cohesion of raw soil and lime-treated soil increased and the corresponding angles of internal friction decreased sharply while the cohesion and angles of internal friction of fibre-reinforced soil varied a little with increase of aggregate size;(3) after failure in compression test,samples of raw soil took on strain-softening ductile failure,those of fibre-reinforced soil took on strain-hardening ductile failure,whereas those of lime-treated soil took on brittle failure;(4) the unconfined compressive strength of all studied soil decreased with increase of aggregate size.On the basis of the above findings,the influence of aggregate size on the strength of studied soils was analyzed and discussed.The conclusions drawn from the test were significant and useful for further studies on the engineering properties of filled soils.