Influence of gas radiation on a thermal plume initiated by a linear heat source in a confined cavity is investigated through numerical simulations. A 2D pure convection case is first considered to validate the numerical code by comparing the critical Rayleigh number against literature results. Then, simulations are extended to 3D con­ ars much earlier for 3D case. Periodic solutions in time exhibit a stationary plane wave which is further broken in the chaotic regime. Finally, gas radiation is introduced at Ra = 1,2 10**7 by considering different gaseous media: on one hand, a gray gas model with various optical thicknesses, and on the other hand, a real gas model for humid air (air - H 2 O mixture). Results show a strong influence of the radiative transfer on flow regimes. By increasing the optical thickness, radiation tends to stabilize the plume and delays the onset of unsteadiness. Comparison of the time-averaged temperature and velocity distributions for the considered gas models indicates that gas radiation reduces the spatial spreading of the plume but has little effects on the kinetic field.