Abstract We present a statistical study of the C i ( 3 P 1 3 P 0 ), C i ( 3 P 2 3 P 1 ) lines (hereafter C i (1–0) and C i (2–1), respectively) and the CO(1–0) line for a sample of (ultra-)luminous infrared galaxies ((U)LIRGs). We explore the correlations between the luminosities of CO(1–0) and C i lines, and find that correlates almost linearly with both and , suggesting that C i lines can trace total molecular gas mass, at least for (U)LIRGs. We also investigate the dependence of / , / , and / on the far-infrared color of 60-to-100 μ m, and find non-correlation, a weak correlation, and a modest correlation, respectively. Under the assumption that these two carbon transitions are optically thin, we further calculate the C i line excitation temperatures, atomic carbon masses, and mean C i line flux-to-H 2 mass conversion factors for our sample. The resulting masses using these C i -based conversion factors roughly agree with those derived from and CO-to-H 2 conversion factor.