In response to the difficulty of the synthesis of high-entropy carbides (HECs), a pressureless low-temperature sintering strategy was utilized to prepare (Zr,Hf,Nb,Ta)C powders with three different transition metal proportion, which powder mixtures of ZrC, HfC, NbC, TaC and Ni were initially annealed at 1 500 ℃ under a flowing Ar atmosphere, and then etched by the nitric acid solution in order to dissolve the metal matrix. The results show that all samples exhibit cuboid morphology with an average size of 2 μm and the exposure of (100) face. Due to high dielectric constant, (Zr1/4Hf1/4Nb1/4Ta1/4)C microcuboids show good microwave absorption performance, and its minimum value of reflection loss attains -48.86 dB at a thickness of 3.5 mm and a frequency of 6.16 GHz. As-fabricated HECs samples also possess good oxidation resistance at 800~1 200 ℃. Moreover, oxidation products are irrelevant to the transition metal proportion and oxidation temperature, which are composed of orthorhombic Nb2O5-Ta2O5 solid solution (o-(NbxTa1-x)2O5), monoclinic ZrO2-HfO2 solid solution (m-(ZrxHf1-x)O2) and orthorhombic HfO2(o-HfO2). Notably, the synergy effect caused by transition metal elements significantly influences the phase transition process of oxides in comparison with monocarbide. In specific, the existence of Hf inhibits the transition of Nb2O5 from orthorhombic to monoclinic, in the meantime, assists the transition of ZrO2 from tetragonal to monoclinic even at 800 ℃. Moreover, the addition of Nb and Ta accelerates the phase transition of HfO2 from monoclinic to orthorhombic under atmospheric pressure.