Peel the sweet potatoes and cut them into ½-inch-thick strips.
Put the strips in a bowl.
Add the oil, rosemary and salt, and toss to coat.
Line a jelly roll pan or cookie sheet with aluminum foil or parchment paper (for easy clean up).
Place the coated sweet potatoes in a single layer on the pan and bake at 400F degrees for about 30-40 minutes, turning them over once midway through the cooking time.
When done, the fries will be tender and slightly browned on the outside.