Abstract: When synthesizing gem-grade diamonds with pressless split-sphere high pressure apparatus (BARS), the requirements on relative density of pressure transmitting medium is pretty high.To enhance the efficiency of pressure transmitting, the preparation of a compound medium, ZrO₂-NaCl, is studied through experiments.The influence of relative density on sealing effect and chamfering is analyzed and the pressure transmitting ability is enhanced by compound ZrO₂ with NaCl.It is found that the compound medium has higher transmitting efficiency than pyrophyllites and that only when the relative density is 92%, the apparatus could work safe and stable.It also requires additional sealing edge.In the end, 2-ct VS-level yellow gem-grade diamond single crystal is prepared on self-made BARS with ZrO₂-NaCl as pressure transmitting medium.