ablation resistance is a key property of ultra-high temperature ceramics (UHTCs) for application in aviation and aerospace. The laser ablation test is one of the effective methods to evaluate the ablation resistance of UHTCs. High-entropy diboride ceramics is a new member of the family of UHTCs, and it is of great significance to understand its laser ablation resistance and related mechanisms for the development of new high-performance UHTCs. However, the laser ablation resistance of highentropy diboride ceramics is not known clearly till now. In response to this problem, the laser ablation resistance of high-entropy diboride (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂ (HEB) ceramics and (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂-20 vol % SiC (HEB-20SiC) composite ceramics have been investigated using a CO₂ laser with wavelength of 10.6 μm and spot diameter of 2 mm to heat the surface of the samples upto ultra-high temperatures in this work. The effect of the SiC secondary phase on the hightemperature oxidation and ablation behavior of HEB ceramics has been studied. It is found that the highest surface temperature, the linear ablation rate and the mass ablation rate of HEB composite ceramics after the laser powder density reached 57.3 MW/m² and dwelled 300 s are 2256 ℃, 0.12 μm/s and -0.014 mg/s, respectively. In contrast, under the same laser ablation test condition, the highest surface temperature, the linear ablation rate and the mass ablation rate of HEB-20SiC composite ceramics are 2168 ℃, 0.08 μm/s and -0.007 mg/s, respectively, which are lower than those of HEB ceramics by ~100 ℃, 33.3 % and 50 %. HEB-20SiC ceramics have higher thermal conductivity than HEB ceramics, and the SiO₂ phase produced by the oxidation of the SiC phase melts and evaporates at high temperatures, which takes away part of the heat and consequently effectively reduces the ablation temperature on the surface of the samples so that HEB-20SiC ceramics show better laser ablation resistance than HEB ceramics