Although the detonation phenomenon in hydrogen nitrous oxide mixtures is a significant issue for nuclear waste storage facilities and development of propulsion materials, very limited amount of critical energy data for direct initiation - which provides a direct measure of detonability or sensitivity of an explosive mixture - is available in literature. In this study, the critical energies for direct blast initiation of spherical detonations in hydrogen-nitrous oxide-Ar mixtures obtained from laboratory experiments and theoretical predictions at different initial conditions (i.e., different initial pressure, equivalence ratio and amount of argon dilution) are reported. In the experiments, direct initiation is achieved via a spark discharge from a high voltage and low inductance capacitor and the initiation energy is estimated accordingly from the current output. Characteristic detonation cell sizes of hydrogen-nitrous oxide-Ar mixtures are estimated from chemical kinetics using a recently updated reaction mechanism. A correlation expression is developed as a function of initial pressure, argon dilution and equivalence ratio, which is fitted to provide good estimation of the experimental measured data. The direct link between cell size and critical energy for direct blast initiation is then analyzed. Good agreement is found between experimental results and theoretical predictions, which make use of the cell size estimation correlation and the semi-empirical surface energy model. The effects of the initial pressure, equivalence ratio and the amount of Ar dilution on the critical initiation energy H-2-N2O-Ar mixtures are investigated. By comparing the critical energies with those of H-2-O-2-Ar mixtures, it is shown that H-2-N2O mixtures are more detonation sensitive with smaller initiation energies than H-2-O-2 mixtures at the same initial pressure, equivalence ratio and amount of argon dilution, except for higher diluted condition with amount of argon in the mixture above 20%. Attempt is made to explain the critical energy variation and comparison between the two H-2-N2O-Ar and H-2-O-2-Ar mixtures from the induction length analysis and detonation instability consideration. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.