Abstract The flame structure at small Damköhler number and large Karlovitz number is still unknown. In the present study, the reaction zone structure in a distributed reaction zone regime was investigated extensively. The OH–PLIF images suggested that there are no thin laminar flamelets and that the reacting eddies were distributed throughout the reactor. With the cross-correlation of ion signals, the scale of the reacting eddies was determined to be of the order of 4 mm, and the convection velocity of these eddies or zones was found to coincide with the mean flow velocity of the order of 100 m/s. In this combustion regime, high intensity combustion can be realized with extremely low NO x emission. These features are very attractive for the practical use (for example, vehicles and aircrafts) to meet the environmental requirements.