We evaluated an appropriate region of interest (ROI) size for the measurement of full width at half maximum (FWHM) in the bead method (0.1 mm and 0.5 mm diameter; lead) and the microdisk method (0.05 mm thickness and 1.0 mm diameter; tungsten) using multislice computed tomography (CT). The FWHM of preset slice thicknesses 0.625 mm, 1.25 mm, 5.0 mm and 7.5 mm were measured by varying helical pitch, location of measurement [center and off-center of scan field of view (SFOV)] and ROI size, and they were compared with the tolerance stated in the Japanese Industrial Standards (JIS). It was conlcuded that the appropriate ROI size was influenced by preset slice thickness in this study. At the center of SFOV, measurements of FWHM were enabled within the tolerance of the JIS with small variations in all preset slice thicknesses if the ROI sizes were set between 0.4 times and equal to the size of the bead or microdisk indicating the maximum CT value in the series of CT images. At the off-center of SFOV, the tendency of increasing FWHM was confirmed, but it was shown that variations of the off-center in thicker slice thickness were larger regardless of helical pitch when the orbital synchronized helical scan technique was not used.