Abstract: This paper presents size effect in the strength of cemented backfill in laboratory testing.The result of test showed that:when concentration and tailings cement mass ratio remained in consensus,the cubes were observed to produce consistently higher strengths than the cylinders,while the 50.8mm×101.6mm cylinders samples possessed less strengths than the 76.2mm×152.4mm samples.Using the method of linear approximation,two size effect conversion functions of the strengths between the cubes and the 50.8mm×101.6mm cylinders,and also between the cylinders were established,which can be a new trigger point of researching size effect in strength of cement backfill and a benefit of the testing standards unification.