Cerium-doped transparent Y3Al5O12 (YAG:Ce3+) is the most widely used phosphor for converting blue to white light in light-emitting diodes (LEDs). Here, YAG:Ce3+ powders were synthesised through microwave-assisted combustion by using organic fuel urea. In YAG system applications, the powder structure and properties play key roles in the performance of terms of white LEDs (WLEDs). Accordingly, this study aimed to determine the effects of thermal treatment and Ce3+ concentration on the structure, morphology, chemical composition and luminescence properties of YAG powders. A direct conversion from the monoclinic and hexagonal phases to the cubic one was achieved at > 850 degrees C. This conversion was accompanied by increased particle size in the range of 50-100 nm and decreased lattice strain. However, powder sintering at high temperature was required to incorporate Ce3+ into the YAG matrix and eliminate residual impurity phases. The highest electroluminescence (EL) emission was achieved for the sintered powders at 1050 degrees C with low dopant concentration (2.0 mol%), offering daylight WLED with a correlated colour temperature of 6834 K and corresponding colour-rendering index of 68.2. Finally, a slightly decreased intensity and red-shift in the EL peak position were observed. These findings were attributed to the presence of a separated CeO2 cubic phase (Ce4+) and concentration quenching for YAG ceramics with > 2.0 mol% cerium content.