Abstract: In this paper a theoretical analysis of heat transfer characteristics is presented for the fully developed laminar flow of the incompressible gas in the triangular microchannels in slip flow regime.The energy equation with the temperature jump boundary conditions is solved by using a computation-oriented method of the orthonormal function analysis in the triangular microchannels heated with uniform heat flux along flowing direction and with isothermal walls. The theoretical calculations of dimensionless temperature profiles and the average Nusselt number in the microchannels are obtained. The results show that the orthonormal function method is applicable to study the heat transfer characteristics in the triangular microchannels.The average Nusselt number in triangular microchannels is lower for slip flow than that for no-slip flow, and decreases with increasing Knudsen number. The relation curve of average Nusselt number vs the aspect ratio of the triangular microchannels moves downward parallelly with increasing Knudsen number.