Clean the herb. It is best to choose sprigs of rosemary that do not require washing. If you must clean your herbs, rinse them off and dry them thoroughly before using.
Slightly bruise therosemary. Bend or gently rub the rosemary to release the wonderful scent of the herb.
Heat the oil. Place the rosemary into a medium saucepan and cover it with oil. Heat the oil over medium-low heat to 150°F/66°C. Be careful not to exceed the temperature too much or it can burn the delicate flavors of both the oil and the herb. Heat for 10 minutes. The oil should be warm, but not simmer. Turn off the heat and let the rosemary infuse in the oil for 1 hour.
Strain the oil. Using a fine-mesh strainer, strain the oil into a clean, dry glass bottle (it is not necessary to sterilize the bottle).
Store. Herb-infused oil keeps well away from direct light or heat at room temperature for up to 2 months, or in the refrigerator for 6 months. While this oil can be safely stored at room temperature, oil flavor quality is maintained for a longer period of time when refrigerated. It is also best to protect infused oils from light by storing them in dark-colored bottles.