• Three dense bulk high-entropy REMgAl 11 O 19 and LaMAl 11 O 19 ceramics are designed and successfully prepared via solid state reaction in a single step. • The factors affecting the phase stability of LaMgAl 11 O 19 are firstly investigated via the electronic structure. • Enhanced thermal stability and infrared emissivity properties are demonstrated for the first time. • Mechanism for excellent thermal stability and infrared emissivity properties is revealed for the first time. LaMgAl 11 O 19 (LMA), characterized by high melting point, low density and thermal conductivity as well as good infrared emissivity, is regarded as a potential candidate for the thermal protection of hypersonic vehicles. Nevertheless, the unsatisfied phase stability at high temperature results in declining of the emissivity below 6 μm, which limits the extensive applications of LaMgAl 11 O 19 . In order to overcome this obstacle, three dense bulk high-entropy ceramics, (La 0.2 Nd 0.2 Gd 0.2 Sm 0.2 Pr 0.2 )MgAl 11 O 19 (HE LMA-1), (La 0.2 Nd 0.2 Gd 0.2 Sm 0.2 Dy 0.2 )Mg Al 11 O 19 (HE LMA-2) and La(Mg 0.2 Fe 0.2 Co 0.2 Ni 0.2 Zn 0.2 )Al 11 O 19 (HE LMA-3), were designed and successfully prepared through solid state reaction at 1700 °C for 4 h in one step. XRD analyses show that the phase compositions of HE LMA-1, HE LMA-2 and HE LMA-3 are single-phase solid solutions with the relative density of 95.61%, 95.49% and 94.31%, respectively. Heat treatment experiments demonstrate that the phase composition of HE LMA-1 remains a single phase after high-temperature heating, while second phase appears in other two samples. The stability of HE LMA-1 is attributed to small average size difference δ (∼4%) of constitute elements. Intriguingly, the average emissivity of HE LMA-1 in the range of 3–6 μm reaches 0.9, which is significantly higher than that of LMA and other two HE LMA samples. The emissivity of all samples remains above 0.95 from 6 to 10 μm. In the far infrared region (10–14 μm), although the emissivity of these specimens decreases slightly, it still exceeds 0.85. The UV–Vis absorption spectra indicate that the formation of many discrete impurity energy levels with small intervals in HE LMA-1 promotes the f electrons to transit between adjacent impurity energy levels and conduction band, which enhances the infrared emission of HE LMA-1 below 6 μm. In a word, with improved phase stability and thermal emissivity in infrared range, high-entropy REMgAl 11 O 19 , especially (La 0.2 Nd 0.2 Gd 0.2 Sm 0.2 Pr 0.2 )MgAl 11 O 19 (HE LMA-1), is a promising candidate in thermal protection coatings of hypersonic vehicles.