In this paper, the quantity of dot gain in off-set printing is estimated by using the method of UCR(under color removal) and GCR(gray component replacement) and the degree of dot gain is researched through measurement of dot coverage of each color patch at the output film that is variously applied to discretionary quantity of dot gain each line in screen in the printing for the process of color separation and at the offset printing. Also, the best appropriate quantity of dot gain treatment is examined by printing each line in screen for reproduction of colors.