The difference between the ITS-90 and the IPTS-68 in the temperature range from 630°C to 960°C is determined comparing an HTPRT and five Pt-Rh 10%/Pt thermocouples. The values of temperature based on the ITS-90 are obtained with an HTPRT calibrated at the freezing points of silver, aluminum, zinc and tin, and the triple point of water. Those of the IPTS-68 are obtained with thermocouples calibrated at the freezing points of gold, silver and aluminum. The HTPRT and the thermocouples are intercompared using a gas controlled sodium heat pipe, the temperature of which is stable within±10mK. The difference between the ITS-90 and the IPTS-68 is estimated from the intercomparison and is found to be 20mK±50mK near 800°C. The largest error in determination of the difference is attributed to the uncertainty of the thermocouples.