As a thermal interface material, thermal grease (TG) has been extensively applied to facilitate heat dissipation in electronic devices. Despite the superior thermal conductivity of diamond, researches on diamond-containing TGs remain rare. In this study, four kinds of TGs in which diamond served as essential filler were prepared and hot disk technique was applied to measure their thermal conductivity . After two unoverlapped particle sizes were selected, the volumetric filler content, terminal group, and viscosity of a polydimethylsiloxane (PDMS) matrix were modified in sequence. Based on the preferred recipe of a single-filler TG, two double-filler TG series were prepared by retaining the large diamonds and replacing the small ones by or ZnO, respectively. Depending on the content, it was found that diamond was not always the best choice for small filler. The highest , which was 23 times greater than the original , appeared in a ZnO-containing double-filler grease . The prediction for the maximum attainable thermal conductivity was preliminarily supported.