Grinding balls are an indispensable grinding medium in vertical grinding mills, grinding ore powder to target size. Some were severely worn and unrounded by iron ore from Anshan, China, during grinding operation in the vertical grinding mill (VTM-1500, Metso Outotec). Their service failure behavior was explored, based on classifying the wear shapes of the recovered grinding balls, combined with the ore-media operating behavior. The wear mechanism of the unroundness, invagination and the special-shaped reduction was verified by the three-dimensional (3D) wear sizes and position relationships of grinding balls. The main wear model of grinding balls was polygon wear, including pentagon shape and quadrangle shape. The minor wear form was abnormal wear, containing biconvex spindle contour wear and single pit contour wear induced by free wear distribution. Both mainly occurred on the packing protective surfaces, resulting in several normalized final shapes along with the wear progress. This investigation provides a theoretical basis for understanding the interaction behavior among slurry and grinding balls.