While heat transfer in Taylor (slug) flows in circular microtubes has been examined exclusively by researchers, noncircular microchannels have not been widely considered in the literature. This is a large gap in research since noncircular microchannels are common structures in microcooling processes. Square and rectangular microchannels are the most important examples. In the present study the heat transfer process in slug flows in square microchannels has been investigated numerically under constant wall temperature boundary condition. The local heat flux for the moving slugs has been converted to total microchannel heat flux using the integration methods suggested recently by the authors. This leads to microchannel wall average heat flux which is the parameter of interest in heat sink problems. Potential effects of liquid film around bubbles on heat transfer process have been discussed by comparing the numerical results with experimental data. Finally, effects of Reynolds number, contact angle, and slug length on heat transfer in slug flow inside square microchannels have been studied and graphs for prediction of slug flow heat transfer in square microchannels have been provided for slug flows with no liquid film or very thin liquid film around the bubbles. (C) 2013 Elsevier Ltd. All rights reserved.