Diamond/liquid metal composites hold significant promise for applications in thermal management. In this work, the effect of variation in surface roughness and sp2 carbon for diamond particles using different surface treatments on the thermal conductivity of diamond/InSnBi composites is systematically studied. AFM characterization shows that the surface roughness of the diamond increased by 2.74–6 times after air annealing. The XPS and Raman spectra confirm that the sp2 carbon is formed on the surface of the air-annealed diamond. With subsequent treatment of acid oxidation, the diamond exhibits a removal of the sp2 carbon and a slight increase in surface roughness. Compared with the InSnBi composite with the as-prepared diamonds, the composites with the air-annealed and air-acid treated diamonds exhibit an increase in thermal conductivity up to 60.37 and 65.44 W·m−1 K−1, which is higher than other composites with coating an interfacial layer. Such thermal conductivity improvement is attributed to the increase in diamond surface roughness, which enlarges the interfacial contact area for heat transfer. Additionally, the increase in diamond surface roughness enhances the wettability and interfacial bonding between diamond and InSnBi, which reduces their interface thermal resistance. These findings underscore the crucial role of increasing diamond surface roughness and the subsequent removal of sp2 carbon in enhancing the thermal conductivity of diamond/InSnBi composites. Display omitted •The surface treatment of air annealing followed by acid oxidation leads to the increase in surface roughness of diamond.•The diamond/InSnBi composite with diamond surface treatment exhibits an increase of 1.81 times in thermal conductivity.•The improvement of the thermal conductivity is attributed to the increase in diamond surface roughness.