Coherent structures were experimentally investigated in a heated air jet vertically ejected from a round nozzle of 30mm in diameter. Some experiments of the present jet, mean velocity at the exit U_a=1.5〜5.0m/s and temperature difference ΔT=0〜100K, were conducted by smoke wire method and smoke method. Coherent structures, vortex pairing and side jet were observed. The side jet occurred in the jet with ΔT=75, 100K and ρ/ρ_∞=0.74〜0.8.