The microchannel flow in miniature TCDs (thermal conductivity detectors) is investigated numerically. Solutions based on the boundary-layer approximation are not very accurate near the channel inlet for low Reynolds numbers. As a result the full Navier-Stokes equations were solved to analyze the gas flow in a miniature TCD. The effects of channel size and inlet and boundary conditions on the heat transfer rate were examined. When the gas stream is not preheated, the distance for a miniature TCD to reach the conduction-dominant region is approximately two to three times the thermal entry length of a constant property pipe flow subject to a uniform thermal boundary condition. If the gas inlet temperature is in the vicinity of the mean gas temperature in the conduction-dominant region, the entrance length is much shorter and very close to that of a constant property pipe flow with uniform surface temperature or heat flux.