Flame structure of diffusion-controlled semi-spherical flame (we call “microflame”) is investigated numerically with Cl chemistry reaction model. Axisymmetric 2-D mass/heat transport processes and chemical processes (including elementary reaction steps) are considered in this study. Heat conduction and surface radiation of the burner is taken into account to predict the thermal interaction between burner and flame. Effects of thermal condition of the burner wall, preheat and flame orientation (normal and inversed direction) on flame size are discussed. Flame structures indicate that apparent flame base is formed and fuel conversion from CH4 to CO and H2 occurs in the fuel side. It is found that the key reaction at flame base is the chain branching reaction (H+O2→OH +O), and most of other important reactions contributed on flame miniaturization are triggered by this reaction. Fuel preheating is naturally involved due to the thermal input from both radial and axial direction. Preheating oxidizer is much effective for the miniaturization through the chain branching reaction enhancement. Premixing is also powerful way to minimize the flame size. Comparison between normal and inversed microflames (jet direction is opposed/concurrent to the gravity vector) shows that flame base structure and mechanism are essentially held irrespective of the flame orientation.