The problem of the crystallite size determination for nanomaterials from X-ray diffraction data obtained in asymmetrical GIXD geometry was analyzed. The studies were performed on nanocrystalline MgO powder prepared by sol-gel synthesis. The nanopowder was preliminary characterized from X-ray diffraction pattern registered in classical Bragg-Brentano geometry and electron microscope observation. The estimated crystallite size, calculated form Williamson-Hall method, equals to 5 nm whereas the lattice distortion is negligible (0.1%). The X-ray diffraction patterns were registered in 30-135º 2θ range using tunnel GIXD technique for the incident α angle: 0.25; 0.5; 1; 2.5 and 5 degrees, respectively. Additional broadening of diffraction lines originated from applied geometry was observed. The calculated crystallite size deviate significantly in comparison to results obtained from classical Bragg-Brentano data. Corrections for additional line broadening were determined, which should be applied for accurate crystallite size calculation in studies of thin nanocrystalline layers using GIXD technique.