TiO2 nano-particles (NPs) with an average primary size of 30 nm were fluidized by nitrogen and air in a bench scale conical vessel. The NPs tended to form simple-agglomerates with porous structures because of weak physical inter-particle forces. When the NPs were fluidized, they agglomerated further to form complex-agglomerates with coral-like structures, several hundred microns in size. A mechanism is proposed for the formation of these structures based on the experimental results. Agglomeration analysis by a laser technique and TEM imaging was used to measure the size of agglomerates. Bed collapse tests and bed expansion experiments were performed in the bed. The pressure-drop and bed height, as well as the gas velocity, were compared for NPs and micro-particles. The average size of agglomerates was estimated by both fractal analysis and based on the modified Richardson-Zaki equation. Model predictions are mostly in good agreement with the experimental data.