The shape, size, density, and porosity of fluidized nanoparticle agglomerates (FNPAs) at the bottom of a fluidized bed were studied, the internal microscopic structures were revealed by the transmission electron microscopy observation of slices of the solidified FNPAs, and the drag coefficients were measured by the settling experiments. The results show that the distributions of diameter, density, and porosity all follow the Gaussian distribution. They have a similar hierarchical structure to FNPAs in the middle and upper parts of the bed. Primary nanoparticles connect to form aggregates of a few hundred nanometers, which then form simple agglomerates of several micrometers. The total porosities of FNPAs are in the range of 0.92-0.99, and the porosities between simple agglomerates, epsilon(s,out), are in the range of 0.44-0.93. The values of the drag ratio, Omega, are larger than 1, around 1, and lower than 1 when values of epsilon(s,out) are in the ranges of <= 0.58, 0.58-0.77, and >0.77, respectively.