Nanograin size of about 3-5nm of 20mol% Y-doped BaZrO3 (BZY20) proton conducting solid electrolyte has been prepared at low processing temperature (< 200 degrees C) and a bulk nanograined sample has been obtained at room temperature using cubic anvil pressing at 4GPa. The grain morphology and structural changes of the as-pressed and annealed BZY20 samples have been studied using powder X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM) equipped with energy dispersive X-ray (EDX), and electron diffraction analyses. Due to the presence of inhomogeneity such as trace formations of hydroxide phases of the as-pressed sample as observed from thermogravimetric and differential thermal analysis (TG/DTA), Fourier transform infrared (FTIR) results, and from the thermodynamics phase equilibria viewpoint, the conductivity is immeasurable. On the other hand, the sample annealed at 800 degrees C with similar to 10nm grain size shows a rather low protonic conductivity possibly due to poor interfacial grain boundary contacts and structurally disordered interface. However, as the grain grows to a well-crystallized structure with better interfacial and refined gain boundary and better yttrium distribution from grain interior to the grain boundaries, the total protonic conductivity increases and shows a good agreement with the results obtained using proton and deuteron isotope exchange reaction. The total protonic conductivities, bulk and grain boundary contributions, at 500 degrees C for samples annealed at 800 degrees C, 1250 degrees C, and 1500 degrees C are 8.7 x 10(-6) Scm(-1), 2 x 10(-3)Scm(-1), and 4 x 10(-3) Scm(-1) with grain sizes of about 10nm, 50nm, and 200nm, respectively. (c) 2008 Elsevier B.V. All rights reserved.