A powder diffraction method was applied to the quantitative analysis of amorphous silica in several quartz powders. Two calibration methods, i.e., direct analysis and the standard addition method were examined. Calibration mixtures were made by mixing a standard silica gel powder ground to under 5 μm particle size with a matrix quartz powder which was ground to 10 to 40 μm particle size and treated with NaOH solution to remove the amorphous phase caused by grinding. Intensity of the amorphous halo was measured at 23.0° 2θ, and the background intensity at 53.0° 2θ was subtracted. Linear calibration curves were obtained over the ranges of 0 to 50 wt% by direct analysis and 0 to 20 wt% by standard addition methods, respectively. The analytical results obtained by the two calibration methods were in good agreement with each other. The relative standard deviations for 4.3 wt% of amorphous silica were 4.6% by the direct analysis and 5.4% by the standard addition method. These methods were successfully applied to a correction of reference intensity ratios (RIR) for several quartz powders containing amorphous silica. After the correction for amorphous content, the relative standard deviations of the RIR values for quartz powders became smaller.