Laser activation annealing using of an Mg-doped GaN four-point probe small mesa device using a 193-nm ArF excimer laser is investigated. Fabricated mesa device has 2-μm-high small mesa structures with In/Au contacts formed using standard semiconductor device process. In our setup, the ArF excimer laser was directed onto the sample using a series of mirrors and was focused through an aspheric lens to the sample, which is mounted on to a movable stage. The mesa device and the laser were aligned while being viewed on the computer monitor screen using a UV CCD camera. This method has several merits: (1) the four-point probe measurement allows accurate resistivity measurement of the GaN layer independently of the contact resistance, (2) utilizing a small area mesa also as the alignment marker during laser annealing ensures that the irradiated area precisely corresponds to the resistivity measurement area, and (3) multiple irradiation-measurement cycles are possible with a single mesa device, which avoids wafer-scale variations. The dependence of the resistivity to the laser fluence and irradiation time were investigated by multiple-irradiation measurement cycles to a single mesa device in combination with atomic force microscopy analysis for surface morphology characterization. Results reveal that laser annealing at 530 mJ/cm2 for 1500 s (at 150 Hz repetition rate) decreases the resistivity from 11.5 (before irradiation) to 5.6 Ω·cm (after irradiation). This value is similar to resistivity achieved by rapid thermal annealing (RTA) at 800°C for 120 s, suggesting successful Mg-doped GaN activation. Results also show stronger dependence on the irradiation time (temporal dependence) than the laser fluence, which may imply that activation mechanism for the laser annealing has a thermal contribution. This is the first report demonstrating ArF laser annealing that achieves a similar degree of activation with conventional RTA using resistivity measurements.