A method for evaluating the line sharpness in digital printings is established. First of all, a test diagram, with some lines spaced with the same distance as the lines width, orienting in horizontal and vertical direction, was designed. A digital printer was used to output the test diagram on eighteen kinds of paper commonly used in market. From the eight attributes of line quality in ISO/IEC 13660 standard, three attributes, that are contrast, edge raggedness and line width, which were extracted our to be the main index describing the line sharpness. Furthermore, connecting them with the subjective evaluation, an evaluation model for predicting the perceived line sharpness were established, and the model show that the perceived line sharpness was highly correlated with these three attributes, among which quantified contrast exhibits the highest correlation with subjective, while raggedness and width exhibits a similar and weaker correlation. The perceived line sharpness was able to be predicted through a linear model.