In production of manufactured fine aggregate (MFA) from magnetite mill tailings (MT), the micro fines (particles smaller than 75 microns) was strictly eliminated. This results in lower utilization of the resources and worse micro grading of MFA. The mortar samples were prepared with MT at various micro fines contents to investigate the effects on their fluidities and strengths. In optimal water-to-powder ratio (w/p, all powders included), the fluidities of mortars reached maximum and the strengths of mortars could be improved at suitable content of micro fines in MFA. The application proves that MT can be used as MFA to prepare construction mortars up to standard and micro fines can be substituted for mineral admixture to prepare composite mortars for improving workability. So more MT can be utilized and micro fines in the tailings can take effect.