In order to select threshold of CT image of asphalt mixture for image segmentation more accurately, the perlite powder was added in asphalt mixture to increase the density contrast, three different mixture gradations in which added different levels of perlite powder were prepared and compacted in laboratory, the X-ray CT was used to scan the asphalt mixture specimen to obtain continuous CT images, the CT images were transformed to be histograms which formed double peak. Through comparing with the double peak situation of three mixture types, AC-13 has the best double peak situation, AK-13 and SMA-13 have similar feature of histogram. The results indicate that the addition levels of perlite powder influence the double peak situation significantly. This new approach is an effective way to identify aggregates, mastic and air voids exactly.