Computed tomography arterial portography (CTAP) is widely used with a fixed scan timing and contrast medium quantity; however, these parameters are not necessarily optimal. In this study, CTAP scan timing was analyzed by different bolus tracking methods to monitor the inflow of the contrast medium in real-time. A total of 249 patients who underwent CTAP were assessed. In 30 patients, the CTAP scanning began 33 s after contrast medium injection started (fixed method). In 74 patients, the regions of interest (ROIs) were established at two places in the inferior vena cava above the hepatic vein (inferior vena cava-ROI method). In 145 patients, the ROI was established at two places in the liver parenchyma (liver parenchyma-ROI method). Scan timing was considered appropriate when the difference in the CT value between the hepatic and portal veins approached 0; this was observed significantly more with the liver parenchyma-ROI method than with the other methods. CTAP scan timing with the liver parenchyma-ROI method was better than that with the fixed and inferior vena cava-ROI methods.