The L-shell radiative opacity of lowly charged Cu plasmas is investigated using a detailed level accounting method. The transmission spectra are compared with a recent experiment at ∼16 eV and 0.005 g/cm3, and good agreement is observed. For a systematic study, radiative opacities caused by 2p → 3d transitions at temperatures of 10–35 eV and densities of 0.001–0.1 g/cm3 are calculated. The dominant ionization stages are lowly charged ones with an open M-shell at such plasma conditions. The result shows that charge state distribution and radiative opacities are very sensitive to temperature. The two strongest absorption peaks of 2p3/2 → 3d5/2 and 2p1/2 → 3d3/2 caused by relativistic orbital splitting are well separated at temperatures lower than 25 eV, whereas they are mixed together to form a broadband structure at higher temperatures.