We have investigated the optimal growth conditions for GaN/AlxGa1 - xN multiple quantum well (MQW) structures by metal organic chemical vapor deposition. Optical properties of a set of GaN/AlxGa1 - xN MQW samples grown under systematically varied growth conditions have been studied by employing picosecond time-resolved photoluminescence (PL) spectroscopy. The PL emission efficiency, the linewidth of the PL emission spectra, the ratio of the barrier emission intensity to the well emission intensity, and the temperature dependence of the PL decay lifetime of these GaN/AlxGa1 - xN MQW structures have been measured and compared with each other carefully. Based on our studies, we concluded that the optimal growth conditions for GaN/AlxGa1 - xN MQW structures are GaN-like rather than AlxGa1 - xN-like or other conditions. The GaN/AlxGa1 - xN MQW structures grown under the GaN-like growth conditions exhibited higher quantum efficiencies and narrower PL emission linewidths than those grown under other conditions. PL emission from barrier regions was not observed in the MQW structures grown under the GaN-like growth conditions, which is highly preferred for ultraviolet light emitter applications.