The steady shear flow behavior of the electro-rheological (ER) nano-suspensions with different volume fractions of particles (φ) was investigated under no external electric fields. The apparent yield stresses were found for the samples with φ = 2.4 and 8.8 vol%, and they were discussed with the results of the apparent viscosity as well as the absolute values of the complex viscosity. Macroscopically developed microstructures of secondary particles were observed for the two samples in the quiescent state. On the other hand, no plateau region was measured for the sample with φ = 1.2 vol%, and secondary particles on the order of several micrometers were observed to be separately distributed. The apparent yield stresses were also discussed in relation to the microstructures for the nano-suspensions. It was proposed that randomly distributed secondary particles or a macroscopically developed microstructure of secondary particles in the nano-suspension should also be taken into account for the initial state of the ER effect, other than randomly distributed primary particles generally assumed so far.