This research aimed to determine the change of cutting knives decision depending on knife wear phenomena, anditseffect on hammer mill productivity and product quality. It can be done by tracing the knives wear stages and determine its safe time range to replace the knives. The experiments were done in El-Serw village Damietta Governorate from 2014 to 2016, by grinding both of corn ear and dough as popular agricultural productive using the traditional hammer mill. The studied variables also are four concaves hole diameters (2.5, 4.0, 8.0 and 12.0 mm), five operating hours of new knife, 150, 200, 250 and 300 h for corn ear. While, for dough, are used two concaves hole diameters (2.5 and 4.0 mm) and operating hours of new knife, 200, 400, 600 and 800 h. The measurement includes specifications of knives, product quality and hammer mill performance. The research recommended that the hammer mill knives must replacement after 220 operating hours to grind corn ear and 600 operating hours to grind dough which represents the safe limit time to keep product quantity and quality in accepted state.