A radiation source apparatus is provided. The radiation source apparatus includes a chamber, a target droplet generator, an exhaust module, a measuring device, and a controller. The target droplet generator is configured to provide a plurality of target droplets to the chamber. The exhaust module is configured to extract debris corresponding to the target droplets out of the chamber according to a first gas flow rate. The measuring device is configured to measure concentration of the debris in the chamber. The controller is configured to adjust the first gas flow rate according to the measured concentration of the debris.