Because the allowable current loading of buried electrical transmission cables is frequently limited by the maximum permissible temperature of the cable or of the surrounding ground, there is a need fur cable backfill materials that can maintain a low thermal resistivity even while subjected to high temperatures for prolonged periods. Temperatures greater than $50^{circ}C;to;60^{circ}C$ may lead to breakdown of cable insulation and thermal runaway if the surrounding backfill material is unable to dissipate the heat as rapidly as it is generated. This paper describes the results of studies aimed at the development of backfill material to reduce the thermal resistivity. A large number of different additive materials were tested to determine their applicability as a substitute material. Tests were carried out for Dongrim river sand, a relatively uniform sand of very high thermal resistivity, $50^{circ}C-cm/watt;at;10\%$ water content, $260^{circ}C-cnuwatt$ when dry, and Jinsan granite screenings, and D-2 (sand and granite screenings mixture), E-1 (rubble and granite screenings mixture), a well-graded materials with low thermal resistivity, about $35^{circ}C-cm/watt$ when at 10 percent water content, $100^{circ}C-cm/watt$ when dry. Based on this research, 3 types of backfill materials were suggested for improved materials with low thermal resistivity and the applicability was assessed through field tests.