Here, Cs4PbBr6 microcrystals (MCs) are synthesized using a two-phase liquid-liquid immiscible method. In order to unravel the bright green photoluminescence (PL) mechanism for in-situ Cs4PbBr6 MCs, the thermal stability of PL spectra for the supernatant and precipitate of reactants is investigated comparatively. Exciton binding energy, exciton-phonon (EP) coefficient and PL lifetime all indicate that the PL of the precipitate has similar temperature dependence as that of the supernatant. It is found that, according to its structural and optical characteristics, the supernatant of the reactants is CsPbBr3 nanocrystals (NCs). We also find that the bright green PL from the precipitate of the reactants is due to CsPbBr3 NCs embedded into Cs4PbBr6 MCs. Experimental results further reveal the size of CsPbBr3 NCs embedded into Cs4PbBr6 MCs is larger than that of CsPbBr3 NCs in the supernatant, The surface passivation of the composites can thus help to suppress the thermal quenching of PL for CsPbBr3 NCs, which opens a new avenue for enhancing the thermal stability of PL for perovskite NCs. (C) 2019 Elsevier Inc. All rights reserved.