A method in which, during an optimal power calibration (OPC) process, an optical disk drive performs a plurality of write tests to an optical disk at a plurality of test powers and measures a corresponding plurality of modulation signal strength values of the optical disk for the plurality of write tests to generate a modulation signal strength versus power curve. The method then determines possible gamma lines corresponding to considered powers, and selects a considered power as a target power of the optical disk drive when a possible gamma line is substantially tangential to the modulation signal strength versus power curve at the considered power. According to the method, powers within a domain of the plurality of test powers are considered until a considered power is selected as the target power for the optical disk.