This work investigated transient processes in gas-phase combustion over a porous sphere. According to a numerical model, the ignition criterion,dg/dt = 0, at the spherical surface is verified to be appropriate for the entire flow field that includes both the front and wake regions of the sphere. The flame developed stage is defined to be when the surface heat flux becomes invariant with time for all orientations. The effects of Damkohler and Lewis numbers on the ignition delay and the period of flame development are examined. For a small Lewis number or a large Damkohler number, ignition initiates near the forward stagnation point, and a stable enveloped flame is formed. Conversely, for a large Lewis number or a small Damkohler number, ignition takes place in the wake region and the developed flame is a wake flame.