A laser crystallization apparatus includes: a plurality of laser generators which generates an incident laser beam; an optical system which optically converts the incident laser beam to an output laser beam; a process chamber in which a thin film formed on a substrate is crystallized by the output laser beam radiated thereto; a first monitoring device which detects a synthesized pulse of the output laser beam; a second monitoring device which detects individual pulses of the incident laser beam; and a controller which controls oscillation times of the plurality of laser generators. The controller generates a plurality of synthesized pulses by combining the individual pulses of the incident laser beam, and derives an optimal synthesized pulse from the plurality of synthesized pulses.