Purpose. To investigate the impact of diamond rotary instrument grit size and rotational speed on cutting efficiency through lithium disilicate and zirconia ceramic materials. Material and methods. A rectangular-shaped specimen of 12.5 × 14.5 mm and 2 mm thickness of forty lithium disilicate (IPS Emax; Ivoclar) and forty 4 mol% yttria zirconia (Katana STML; Noritake) was milled and sintered according to the manufacturer's instructions. Two diamond rotary instruments (X-tra coarse, 230 µm grit size, and fine, 46 µm grit size; Komet USA) were used to cut into the ceramic materials at 200 000 and 50 000 revolutions per minute (RPM) under a continuous water cooling rate of 50 mL/min (n = 10/grit/speed). Handpiece movement was controlled, torque was maintained at 3.4 Ncm, and pressure was maintained at 2 N using a load sensor. The efficiency of each rotary instrument was calculated. Scanning electron microscope images were made of each rotary instrument to visualize wear and deterioration. Data were analyzed statistically (α = 0.05). Results. Ceramic material cutting efficiency differed amongst diamond rotary instruments (P<.001). A coarse diamond rotary instrument at 200 000 RPM cut both ceramics efficiently (P <.001)for zirconia and lithium disilicate specimens; cutting efficiency was .064 and .107 mm/sec, respectively. Only coarse diamond rotary instruments can cut 10 mm in 10 minutes. High speed led to more efficient cutting regardless of ceramic material or rotary instrument grit (P<.001). Scanning electron images show higher wear and degradation in both grit diamond rotary instruments at 200,000 RPM than at 50,000 RPM. Conclusions. Coarse-grit diamond rotary instruments cut zirconia and lithium disilicate ceramics more efficiently than fine-grit ones. High-speed 200 000 RPM cuts ceramics more efficiently than 50 000 RPM at a lower speed.