Since ITS90 was established, the whole temperature range of practical importance has been covered by the inter- nationally approved temperature standard’. The standard is realized by defining fixed points with assigned temperature values and devices to interpolate between them. Working thermometers are then calibrated against the standard thermometers, following various kinds of cali- bration procedures. A typical calibration system is com- posed of two processes; the hardware and the software pro- cedures. There are many useful reports on the hardware procedure2,3. Here the software procedure, including cali- bration point temperatures, interpolation functions and fit- ting accuracy, is discussed. In particular, this report focuses on deducing optimum calibration procedures applicable to any kind of temperature sensor. Temperature sensors are conventionally characterized by their sensitivity (or working temperature range), stability and applicability for special conditions in terms of physical dimensions, response time, use under magnetic fields or irradiation, etc. The present authors propose a different aspect for characterization; ‘calibration efficiency’, which is a measure of how easy it is to calibrate the sensors using a small number of calibration points, with small residual errors.