In this paper, the lapping process of wafer surfaces is studied with experiments and contact modeling of surface roughness. In order to improve the performance of the lapping processes, effects of mixed abrasive grits in the slurry of the free abrasive machining (FAM) process are studied using a single-sided wafer-lapping machine. Under the same slurry density, a parametric experimental study employing different mixing ratios of large and small abrasive grits and various normal loadings on the wafer surface applied through a jig is conducted. Observations and measurements of the total amount of material removed, material removal rate, surface roughness, and relative angular velocity are presented as a function of various mixing ratios and loadings and discussed in the paper. The experiments show that the 1:1 mixing ratio of abrasives removes more material than other mixing ratios under the same conditions, with a slightly higher surface roughness. Modeling of the mixed abrasive particle distributions correspondingly indicates that the roughness trend is due to the abrasive size distribution and the particle contact mechanics. The results of this study can provide a good reference to the FAM processes that practitioners use today by exploiting different abrasive mixing ratios in slurry and normal loadings in the manufacturing processes.