how long to cook a prime rib in oven
4 Place the roast fat side up and rib bones down in a roasting pan. Insert a meat thermometer into the thickest part of the roast, making sure that the thermometer isn't touching a bone. 5 After browning the roast at a 500Â°F temperature in the oven for 15 minutes, reduce the oven temperature to 325Â°F.To figure out the total cooking time, allow about 13-15 minutes per pound for rare and 15-17 minutes per pound for medium rare.oast in oven until thermometer registers 115Â°-120Â°F for rare or 125Â°-130Â°F for medium. (The internal temperature of the roast will continue to rise after you take the roast out of the oven.). Check the temperature of the roast using a meat thermometer 30 to 45 minutes before you expect the roast to be done.