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 formulae (1-1) and/or (1-2), one or more kinds of a compound represented by the following general formula (2), and one or more kinds of a compound, represented by the following general formula (3), and/or an equivalent body thereof. 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 as an antireflective film), 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.