This paper studied the indentation size effect (ISE) of non-stoichiometric high-entropy carbide ceramics (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_1-x (HEC_1-x, x=0-0.5) with different carbon vacancy concentration. The experimental results showed that the indentation size increased and the hardness decreased rapidly as the load increased from 0.05 kgf to 1 kgf. Moreover, the proportional specimen resistance (PSR) model can describe the indentation size effect of high-entropy carbides in this study. The hardness decreased gradually with the load rised, but the degree of change decreased first and then increased with the increase of carbon vacancy concentration, which revealed that the ISE in HEC_1-x system had the same trend of variation with the increase of carbon vacancy concentration. The hardness of HEC_0.8 had the smallest change with the increase of load, indicating it had the weakest ISE. According to the fited polynomial curves of load-indentation size relationship, the true hardness of ceramics increased first and then decreased with the increase of carbon vacancy concentration, and the maximum value was 22.2 GPa at HEC_0.9.