There is disclosed a resist underlayer film composition, wherein the composition contains a polymer obtained by condensation of, at least, one or more compounds represented by the following general formula (1-1) and/or (1-2), and one or more kinds of compounds and/or equivalent bodies thereof represented by the following general formula (2). There can be provided an underlayer film composition, especially for a trilayer resist process, that can form an underlayer film having reduced reflectance, namely, an underlayer film having optimum n-value and k-value, excellent filling-up properties, high pattern-antibending properties, and not causing line fall or wiggling after etching especially in a high aspect line that is thinner than 60 nm, and a patterning process using the same.