This work demonstrates an autonomous flight of multiple ornithopters in an indoor flight environment. The tested ornithopters have no on-board sensor for flight state estimation or feedback control. Instead, the indoor flight environment with a motion capture system provides flight state variables by tracking the markers on the ornithopters, and the corresponding control inputs are externally processed and transmitted via a 2.4GHz radio frequency. A feedback controller is designed for each ornithopter to follow a circular path with a desired altitude and radius. To avoid collision between flying ornithopters, the angular speed of each ornithopter is controlled by changing the desired radius of each ornithopter. The ornithopters successfully performed an autonomous formation flight in a confined indoor environment following a commanded circular path with a constant altitude.