We present the results of picosecond time-resolved photoluminescence (PL) measurements for a set of 30 Å well GaN/AlxGa1 - xN (x ∼ 0.2) multiple-quantum-well (MQW) structures with varying barrier widths LB from 30 to 100 Å, grown by metalorganic chemical-vapor deposition. The PL quantum efficiency and the recombination lifetime of these MQWs were observed to increase monotonously with an increase of the barrier width up to 80 Å. These behaviors were explained by considering two distinct mechanisms that control the radiative recombination efficiencies in MQWs. When the barrier width is below the critical thickness, the nonradiative recombination rate increases with a decrease of the barrier width due to enhanced probabilities of the electron and hole wave functions at the interfaces as well as in the AlGaN barriers. On the other hand, the misfit dislocation density increases as the barrier width approaches the critical thickness, which can result in an enhanced nonradiative interface recombination rate. Our studies here have shown that the optimal GaN/AlGaN(x ∼ 0.2) MQW structures for UV light-emitter applications are those with barrier widths ranging from 40 to 80 Å.