Transition frequencies between low-lying energy levels in a single trapped 138Ba+ ion have been measured with laser spectroscopy referenced to an optical frequency comb. By extracting the frequencies of one-photon and two-photon components of the line shape using an eight-level optical Bloch model, we achieved 0.1 MHz accuracy for the 5d2D3/2?6p2P1/2 and 6s2S1/2?5d2D3/2 transition frequencies, and 0.2 MHz for the 6s2S1/2?6p2P1/2 transition frequency.