High-resolution, grazing incidence, X-ray powder diffraction has been used to probe the strain dispersion (microstrain) and diffracting, domain size as a function of depth in alumina and alumina-silicon carbide nanocomposite. The microstrain was high, and of similar magnitude, below ground surfaces in both materials but declined much more rapidly with distance from the surface in the pure alumina than in the nanocomposite. The depth over which significant microstrain was observed was in good agreement with previous measurement of dislocation density by transmission electron microscopy. In neither case was there a systematic trend in diffracting domain size with depth. The strain dispersion at the surface was reduced to that in the bulk of the nanocomposite on annealing, the value being almost constant with depth while the diffracting domain size changed significantly, resulting in a linear decrease as the surface was approached. We suggest that the increase in the strength of the nanocomposite is associated both with a macrostrain oriented normal to the grinding direction and the greater depth of propagation of the microstrain. Further strength increase on annealing must be associated with the observed reduction of both these types of strain.