The wear resistance of a series of PDCs sintered from monomodal and trimodal diamond powders was evaluated by cutting granite on a vertical turret lathe, and the underlying degradation mechanisms were investigated. The results indicate that the wear area is predominantly governed by the cutting force, which escalates with the cutting depth (dc). When dc increases from 0.2 mm to 0.4 mm, the cutting normal (Fn) increases from ~1700 N to 2750 N, therefore the wear mechanism transitions from predominant abrasion to severe spalling. Whatever dc was applied, it is found that the average scratch width increases proportionally with the diamond grain size, which supports that the dislodged diamond grains are likely to act as the primary abrasive media against PCD, rather than the granite debris. The average chipped area is also noted to increase with the grain size. For trimodal PDCs, the most inferior wear resistance occurs at the 44 wt% G 2–4 PDC, where coarse diamond grains are isolated by the fine counterparts. Appropriately increasing the G 2–4 content enhances wear resistance by reinforcing the continuity and robustness of the diamond skeleton. This observation suggests that the wear performance of PDCs is critically determined by the strength of diamond skeleton which could be strategically improved through the optimization of diamond powder size distribution.