A numerical simulation was performed on the heat transfer of mixed convection for fluid flowing across a microtube bundle, using the lattice Boltzmann method. At first, the program code was validated by using a benchmark case of natural convection around a hot single tube inside a square enclosure. The local and averaged heat transfer coefficients of each tube in the bundle with various row numbers were calculated. Numerous cases were simulated, ranging from a weak natural convection case (forced convection) to a pure natural convection case. The results indicate that the total averaged Nusselt number outside the tubes gradually decreases and becomes almost a constant with tube row number at low Reynolds number, which is different from the conventional scaled tube. The averaged Nusselt numbers and temperature fields for various situations were compared. Further influencing factors other than the tube row number on the heat transfer behavior of a tube bundle were also summarized and discussed.