Thermionic emission performance from single crystals of CeB6 and LaB6 was investigated at different operational loads using a test machine, in order to consider CeB6 as a cathode for thermionic RF guns in the future applications. To evaluate cathode quality and uniformity for long lifetime applications, Miram curves in conjunction with their associated practical work function distribution (PWFD) were applied to the thermionic emission results. The analysis of the PWFD plot revealed that the variation of the peak in the work function and full-width at half-maximum for CeB6 is very close to that of LaB6. This suggests that emission performance, uniformity, and expected operational lifetime of a CeB6 cathode are comparable with LaB6 cathodes. In addition, previous simulation results of the back bombardment (BB) effect on CeB6 and LaB6 emphasized that CeB6 presents less sensitivity to BB electrons compared with LaB6 at a 5.5-mu s pulse duration and 8-MW RF power. This gives CeB6 the potential to replace LaB6 as a thermionic cathode for an RF gun. A complete analysis of the cathode emission performance and PWFD at different operational loads for CeB6 prior to a prospective real test is presented.