A color scanning scope USL-21 was used for estimating the subcutaneous fat thickness, the fat percentage and the cross-sectional area of M. longissimus thoracis in live cattle without slaughter. Fourteen fattened Japanese Black steers, eight Holstein steers, and eleven fattened Holstein cows were scanned at the region between the 13th rib and the first lumbar about one week before slaughter. The picture signals obtained from the scanning scope were fed into a computer for rapid calculation of the subcutaneous fat thickness, the area and the fat percentage of the M. longissimus thoracis. For estimating the fat percentage of the M. longissimus thoracis, the operational conditions were fixed (TVG-gain 3, TVG-level 2, TVG-start 0, sensitivity 2). After slaughter, the subcutaneous fat thickness, the cross-sectional area and the fat content of the M. longissimus thoracis were measured at the corresponding carcass region using a measure, a planimeter and the ether extraction method, respectively. The color-scanning estimates of the subcutaneous fat thickness and the cross-sectional area of the M. longissimus thoracis agreed considerably well with the actual carcass measurement in each animal, showing r = 0.85 and r = 0.95 respectively. A high correlation coefficient was obtained between color-scanning estimates based on blue dot percentages and the actual fat percentage of the M. longissimus thoracic figures with fattened Japanese Black steers (r = 0.76), whereas no such a high correlalation was observed with Holstein steers and cows. This result shows that the color scanning scope is a useful instrument for estimating the meat quality of live beef cattle of Japanese Black steers. However, further study is needed to increase accuracy of meat quality estimation.