Densities of compressed CO2 + undecane binary mixtures were measured using a magnetic suspension balance (MSB). Measurements were made at temperatures ranging from 313.15 to 353.15 K and pressures ranging from 8 to 19 MPa at CO2 mole fractions (x(1)) of 0, 0.2483, 0.4641, 0.6797, and 0.8874. The mixture densities were found to increase linearly with pressure and decrease with temperature; this behavior is similar to that of CO2 + decane or dodecane mixtures. The mixture densities show a crossover with composition when the CO2 mole fraction is high. The densities of alkanes increase almost linearly with carbon number, as do the densities of CO2 + alkane mixtures. With the of CO2 mole fraction, the mixture densities increase at first, then decrease. The increase of pressure makes the excess molar volume less negative, whereas the increase of temperature makes it more negative. Densities calculated from the perturbed hard chain equation of state (PHSC EOS) with improved parameters are in good agreement with the experimental densities. Adopting the constant binary interaction parameter k(ij) for different alkanes in the PHSC EOS slightly enlarge the model deviation but simplifies the calculation.