Flame initiation is the process whereby a self-sustained propagating flame front is triggered by an energy source in a reactive mixture. It constitutes a central problem in combustion theory, and its study is of key importance to various combustion phenomena, including flame initiation in combustion engines, accidental explosion of vapor clouds and supernovae explosions. A theoretical and numerical study was conducted to clarify the physical mechanism controlling the flame initiation in mixtures with high Lewis number. The analysis was based on the effects of the curvature and heat addition on flame structure. It was found that the conditions for the existence of a self-sustained expanding spherical flame determine the critical power and energy of the ignition source for successful initiation. A new criterion was established to predict both the critical power and energy. Numerical computations with detailed chemistry of the flame initiation in hydrogen-air mixtures are in agreement with theoretical analysis.