Prepare an outdoor grill or grill pan for medium-high heat.
In a small bowl, stir together the chili powder, brown sugar, cumin, garlic and onion powders, salt and pepper. Set aside.
Lightly oil the grill grates. Brush both sides of the flank steak with oil. Coat both sides with the dry rub. Grill until a golden-brown crust is formed, 5 to 6 minutes per side. Remove the steak from the grill and let rest, loosely covered with foil, for 10 minutes. Thinly slice the steak against the grain. Serve on a warmed platter.