The measurement of the relative X-ray absorption coefficient(CT No.) and the size of the lesion on CT is highly valuable in the diagnosis of the certain disease, guide to treatment planning, and follow-up of patient under treatment. But as the CT is a synthetic image, CT No. and size displayed on CT are greatly influenced by various factors such as kVp, slice thickness, scan time, CT No. of the surrounding geometry of lesion in gantry, window width(W.W) and window level(W.L) etc. The study was performed to evaluate the accuracy and error range of the CT measurement, including CT No. and size affected by various conditions using phantom model designed by author. Author also propose the optimum window width and window level for measurement of real size on CT. The results are as follows; 1. The CT No. of the lesion increase as the CT No. of surround increases if the CT No. of the lesion is below-100 H.U., while the CT No. of the lesion decrease as the CT No. of surround increases if the CT No. of the lesion is above 900 H.U. 2. The CT No. increases as photon influx(mAs) during scan increases. The higher the CT No. of the lesion, the greater the degree of increases are. 3. If the CT No. of the lesion is greater than that of surround, the size of the lesion increases as window level decreases. The narrower the window width, the larger the difference between the maximum and minimum value of measurement and the steeper the slope of change is. 4. The window width affects the size if the mesured size is sufficiently different from the true size. 5. The greater the difference between the CT No. of the lesion and that of surround, the greater the difference between the maximum and minimum value of measurement. And as the CT No. of surround increases, the difference between the minimum and maximum value also increases. 6. The optimal window and level for size measurement are as follows; Soft tissue-soft tissue; W.W 250, W.L 40-60 Soft tissue-fat; W.W 500, W.L- 100-60 oft tissue-bone; W.W 500, W.L- 100-300 Soft tissue-air; W.W 500, W.L- 700-600