Large-scaled and monodispersed ZnO nanospheres have been synthesized successfully using zinc nitrate hexhydrate and triethanolamine (TEA) aqueous solutions as precursors through a facile one-pot microwave-assisted process in the absence of any template. The as-prepared products were characterized by XRD, EDX, FESEM, TEM, HRTEM and PL. The results reveal that solid ZnO nanospheres possess a hexagonal wurtzite structure with diameters of 240-440 nm, which are self-assembled by single-crystal polygonal nanoflakes with 20-30 nm in side lengths. Room temperature photoluminescence (PL) spectrum shows a strong ultraviolet (UV) emission around 387 nm and a broad green band at the center of 530 nm. The possible formation mechanism was also investigated.