Preheat oven to 160°C (325°F). Line 2 large baking sheets with parchment paper.
Slice off top and root ends of beets. Peel. Slice beets into very thin rounds with mandolin slicer or knife.
Lay beet slices on parchment in single layer. Brush tops with olive oil. Sprinkle with salt and pepper. Place baking sheets on middle racks of oven. Bake about 10 min. Turn over beet chips. Return sheets to oven, switching rack positions. Bake about another 5 min. Check and remove any crisped chips to bowl. Continue baking and checking every 2 min. or so, removing chips as they crisp, until batch is done.
To prevent hands from staining, hold beets in clean towel or wear plastic gloves during slicing.