In this study, the effect of cement content (3-7% w/w) on the mechanical strength of the paste backfill was examined at different size of slump (6-7"). The paste backfill samples were prepared using the "clastic" and "spec" ore tailings were subjected to the unconfined compressive strength test at predetermined curing periods. The results showed that the optimum cement content of the paste backfill mixture with a compressive strength of 1.387 MPa at 28 days was 7% w/w at a slump consistency of 7.0" for the "spec" ore tailings. The highest compressive strength of 0.812 MPa at 6.0" slump consistency was determined for the paste backfill prepared from the "clastic" ore tailings when the backfill mixture contained 7% w/w cement. The differences observed in the compressive strength and slump consistency of elastic and spec ore tailings could be attributed to the different characteristics of both tailings such as particle size, chemical and mineralogical composition.