Semi-insulating SiC ceramics is proposed as a new material for InGaAsP/InP laser diode heatsinks. It is featured by high theraml conductivity (2.7 W/cm deg) and high electrical resistivity (4×1013 Ohm cm). The expansion coefficient of the material is 3.7×10-6 deg-1, which is very close to that of InP. The electrical and thermal characteristics, and reliability of InGaAsP/InP buried heterostructure (BH) lasers mounted on SiC heatsinks are also discussed. Stable modulated operation of up to 1 GHz, a maximum temperature for cw lasing of up to 100°C, and an expected median lifetime of 3×104 hrs at 60°C have been demonstrated for these lasers.