To determine the intensity of PAC (plasma arc curing) light compared with conventional QTH (Quartz tungsten halogen) light using a hardness test.The spectral distribution of two light sources was analyzed with a spectroradiometer, after the light intensity was determined. AP-X composite was packed into a black mold (4 mm in diameter and 4 mm in depth) to prepare a cylindrical specimen. The irradiating conditions for the PAC unit in the current study were fixed at 6-second irradiation with 600 mW/cm2 (P600-6), 900 mW/cm2 (P900-6), 1200 mW/cm2 (P1200-6), 1500 mW/cm2 (P1500-6), 1800 mW/cm2 (P1800-6) and 40-second irradiation with 600 mW/cm2 (P600-40). The conventional irradiating condition for QTH was 600 mW/cm2 for 40 seconds (Q600-40). After storage in 37 degrees C water for 24 hours, the hardness of the resin composite was measured with a nanoindentation testing machine. Triplicate readings were made at a distance of every 0.5 mm down to 3.0 mm from the top irradiated surface. The comparison of means was statistically analyzed applying one-way ANOVA and Fisher's PLSD test at a significant level of P< 0.05.Although the peak height of the PAC lamp was different due to the light intensity, the waveforms were parallel with one another and the wavelength of peaks was located at the same number. The hardness value at a depth of 2.0 mm and at values of P1800-6 was not significantly different from the control.