Slip flow and heat transfer in the entrance region of trapezoidal microchannels are investigated numerically. The effects of Reynolds number, channel aspect ratio, channel side angle and Knudsen number on the simultaneously developing velocity and temperature fields, and on the key flow parameters like the friction coefficient and Nusselt number are examined in detail. In the entrance region, very large reductions are observed in the friction factor and Nusselt number due to rarefaction effects, which also extend to the fully developed region, however at much lower levels. An interesting feature associated with the slip flow regime is the existence of the asymptotic inlet values for the heat transfer and friction coefficients. These asymptotic values strongly depend on the Kn, but independent of the geometry, and Re. In the fully developed region, both parameters are also considerably affected by the degree of rarefaction, the aspect ratio and the side angle. Friction coefficients monotonically decrease with increasing Kn, and aspect ratio, while they increase slightly with increasing side angles. Fully developed heat transfer coefficients also decrease with the increasing Kn. However, as the aspect ratio increases their sensitivity to Kn decreases.

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