A series of La1.9Y0.1Mo2O9 bulk samples of different grain sizes were made by microwave sintering the nanocrystalline powders prepared by sol-gel methods. Phases, microstructure, grain size of the powders and bulk samples were examined by using X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM) and scanning electron microscope (SEM); and the electrical properties of the bulk samples were studied by AC impedance spectroscopy. Experimental results show that the substituent Y can stabilize the cubic β phase to room temperature; the bulk samples are dense and uniform with an average grain size from 60 nm to 4 μm; the highly dense bulk samples show enhanced ionic conduction, e.g. the conductivity of the sample with relative density 99% is 0.026 S/cm at 600 ℃, which is two times higher than that of bulk samples prepared by solid-state reaction. It can be concluded that the effect of sample density on the electrical conductivity is mainly due to the grain boundary conductivity; and the effect of sample grain size (from 60 nm to 4 μm) on the electrical properties is not so significant.