Combustion characteristics of hydrogen–air premixed mixture are investigated in 1 mm converging–diverging microtubes with heated wall. Numerical study performed by considering three-dimensional and transient forms of the governing equations and detailed chemical kinetics. Effects of inlet velocity, equivalence ratio and converging–diverging angle are studied on the combustion characteristics. It was founded that the flame position has decisive role in the temperature, and inlet velocity has more impact on the flame position, flame extension and luminous zone at a constant converging–diverging angle. According to the results, when the flame places in the converging–diverging section, increase of the convection heat transfer rate due to the flow velocity increasement and increase of the heat loss due to the surface to volume ratio increasement cause to decrease the flame temperature. It leads to form oscillating flames at some conditions. On the other hand, converging–diverging section plays positive role in flame stability. For cases that the flame places in the outlet section, it removes flame instability near the wall and increases upper flammability limit by increasing heat recirculation in the converging–diverging section. The results showed that converging–diverging microtubes have higher upper flammability limit than the constant cross-sectional one. Also, upper flammability limit increases with the converging–diverging angle.