In this paper, we present a study concerning the special role of the thermal conduction effect on the transition from self-explosion to detonation occurring in the ignition by a pocket of hot reactive gas. At the initial moment, a uniform hot reactive pocket is assumed to be surrounded by cold reactive mixtures. The detailed evolution process from the initial conditions is simulated numerically taking into account chemical reactions, molecular transport effects and compressible effect for one dimensional geometry. It is found that thermal conduction may play a key role in the transition process for the case of intermediate initial temperature of hot pockets. When the initial temperature is just above the thermal extinction limit, a premixed flame is ignited after a diffusive explosion; and when the initial temperature is very high, it is also a premixed flame that is ignited after a constant density explosion. In these two cases, the produced overpressure is much smaller than that produced by the corresponding Chapman-Jouguet detonation. While for a case of initial temperature between these two cases, the thermal conduction effect may establish a temperature gradient leading to the initiation of detonations and the production of a very large overpressure.