A genuine non-destructive imaging of embedded nanoparticles is demonstrated using the novel technique of momentum-transfer X-ray diffraction imaging. For the first time, the method yields direct estimation of the statistical properties of the nanoparticle ensemble. Statistical analysis of the experimental data reveals quantitative information about the size distribution of the diffracting objects. Experimental results defining the statistically average form, orientation and dimensions of metastable Al2Cu (I?a??, I?a?? phases) precipitates embedded in binary Al-Cu solid solution are reported and analyzed.