The so-called low-density steels have generated a lot of interests with their high specific strength and ductility due to the addition of the light element Al. In order to accelerate the low-density steel design, a thermodynamic database has been developed within the present author group. Two Al-containing systems, i.e., Cr–Al–C and Ni–Al–C, were modeled and optimized with CALPHAD approach based on the reliable binary descriptions. For Cr–Al–C system, the Gibbs energy of Cr₂AlC phase was described by a temperature-dependent polynomial with the aid of the experimental data on the heat capacity, instead of estimating heat capacity from Neumann–Kopp rule. The incongruent melting temperature of Cr₂AlC is 1762 K with the invariant reaction of liquid + Cr₃C₂ + Al₄C₃→Cr₂AlC. The phase equilibria between Cr₂AlC and binary phases were well reproduced by using the present model parameters. For Ni–Al–C system, the liquid, fcc and bcc phases have been optimized to fit the carbon solubility in these three phases. A good agreement between the calculated and experimental data has been obtained using the present description of Ni–Al–C system. The reliable descriptions of the two ternary systems developed in the present work can be implemented into the thermodynamic database for low-density steels.