This study investigates the effect of developing flow in a circular tube on the thermophoretic particle deposition efficiency using the critical trajectory method numerically. When both flow and temperature are fully developed (combined fully developed), present results agree with previous theories for the long tube where the flow temperature approaches that of the wall. When the flow is fully developed and temperature is developing, it is found that only near the thermal entrance region (or temperature jump region) of the tube the deposition efficiency is slightly higher than the combined fully developed case (flow and temperature), while the deposition efficiency remains the same for the long tube. When both flow and temperature are developing (or combined developing), the deposition efficiency is about twice that of the combined fully developed case for the long tube and is much higher near the entrance of the tube. Non-dimensional equations are developed empirically to predict the thermophoretic deposition efficiency in combined developing and combined fully developed cases under laminar flow conditions. (C) 2003 Elsevier Science Ltd. All rights reserved.