A light source for EUV is provided. The light source includes a target droplet generator, a laser generator, and a controller. The target droplet generator is configured to provide target droplets to a source vessel. The laser generator is configured to provide first laser pulses according to a control signal to irradiate the target droplets in the source vessel. The controller is configured to provide the control signal according to at least two of process parameters including temperature of the source vessel, droplet positions of the target droplets, and beam sizes and focal points of the first laser pulses. When the average value or the standard deviation of the temperature of the source vessel and the droplet positions of the target droplets exceed the predetermined range, the controller is configured to provide the control signal to the laser generator to stop providing the first laser pulses.