A laser annealing apparatus includes a laser oscillating structure, an oscillator, a beam expanding telescope, a first power meter, and a second power meter. The laser oscillating structure emits a first laser beam of a first wavelength and first beam cross-section to a substrate in a chamber including an optical window. The oscillator emits a second laser beam, of a second wavelength different from the first wavelength, to the substrate. The beam expanding telescope is on an optical path for the second laser beam and expands the second laser beam to a second beam cross-section. The first and second power meters measure energy of the second laser beam and a third laser beam, generated as the second laser beam is reflected by the substrate. The first beam cross-section and the second beam cross-section may be equal.