This work examines the performance of a late mixing porous burner (LMPB) in term of radiant output efficiency. The low-calorific-synthetic-gas from Mangium tree gasification was used as a fuel. The system consists of two main porous media: the first one serves as a fuel preheated porous and the second one serve as a combustor that is around with the air jacket for air preheating and preventing heat loss to the surrounding. The two porous media are separated by the small space which serve as a mixing chamber. The study results indicated that the stabilized combustion of low-calorific-synthetic-gas can be occurred. The radiant output efficiency is increased with increasing equivalence ratio while the radiant output efficiency is decreased with increasing firing rate. Moreover, the LMPB provide the radiant output efficiency in the range of 28.5% to 67%.