The present invention relates to methods for on-line calibrating output power of an optical pick-up. A power adjusting circuit of the optical pick-up has an optical power regulator and an optical power detector. The on-line output power calibrating method includes the steps of: performing a recording pre-process; providing a focus offset value and/or a tilt offset value to the optical pick-up; providing a setting value, corresponding to power under test, to the optical power regulator; detecting laser power emitted from the optical pick-up using the optical power detector; comparing the laser power with the power under test to adjust the setting value of the optical power regulator corresponding to the power under test; and performing an optimum power calibration if the laser power emitted from the optical pick-up conforms to the power under test.