Recently, magnification angiography has become popular, because of improvement in X-ray tubes and development of sensitive filmscreen system. Blood vessel contrast is affected by scattered radiation, MTF of systems and taper effect. This time we introduced one of the new functions called P (ω) composed by content rate of direct radiation, MTF of direct radiation and absorption rate of contrast medium, and tried to obtain prediction of the contrast of blood vessel filled with contrast medium. On comparing the contrast calculated from P (ω) with the contrast of the clinical radiography, we got good result and found that P (ω) was very useful.