In the present work, the heat transfer phenomena in the sensor tube of a mass flow controller (MFC) are studied using both experimental and numerical methods. A numerical model is introduced to predict the temperature profile in the sensor tube as well as in the gas stream. In the numerical model, the conjugate heat transfer problem comprising the tube wall as well as the gas stream is analyzed to fully understand the heat transfer interaction between the sensor tube and the fluid stream, using a single domain approach. This numerical model is further verified by experimental investigation. In order to describe the transport of heat energy in both the flow region and the sensor tube, the Nusselt number distributions at the interface between the tube wan and the gas stream as well as heatlines are presented from the numerical solution. Through this study several assumptions frequently used by previous investigators for their analytical models have been shown to be either irrelevant or physically unrealistic. (C) 2001 Elsevier Science Ltd. All rights reserved.