The present invention discloses a probe assembly for used in an infrared ear thermometer. Given that the exchanged thermal radiation and the infrared detector's temperature are known, the subject temperature can be calculated according to Stefan-Boltzman's law. To make the Stefan-Boltzman's law applicable in a dynamic environment where temperature may vary greatly, the contact temperature sensor (thermistor) must acuurately and fast track the temperature of the infrared detector (thermopile sensor). By using the heat transfer theorem, the disclosed assembly makes the thermistor accurately track the cold junction temperature of the thermopile chip without being in an isothermal condition. Not only minimizes the measurement error in a dynamic environment, the design of the disclosed probe assembly also makes a samll and compact infrared thermometer possible.