The wall effect on heat transfer characteristics was investigated for gas. laminar flow through microchannels. In an extremely thin layer adjacent to the solid wall, we called ＇wall-adjacent layer＇, the change in thermal conductivity of gas vs the distance from the wall surface was derived from the kinetic theory of gases. A model including the variation of thermal conductivity was built to investigate the heat transfer rule. Analytical expressions for temperature profiles and heat transfer coefficients were derived respectively for fully developed laminar flow heat transfer of gases in microtubes and in plain microchannels. The investigation shows that the change of thermal conductivity of gas in ＇wall-adjacent layer＇ would result in significant influence on the heat transfer when passage size is small enough.