Natural bio-nanocomposites (rice husk, sapropel, peat and brown coal) have been studied by TEM, XRD and SAXS. Particle size distributions for the mineral matter in these systems were determined by SAXS with using full contrast technique. In the rice husk silica nanoparticles are forming compact particles with bimodal distribution and mean size is equal 4.7 nm. In the case of peat and brown coal mineral matter in organic matrix are forming more large particles with compact shapes and mean sizes 14.1 nm and 14.3 nm respectively. In the sapropel inorganic matter are forming elongated structures with lengths are equal 20-70 nm and widths are equal 2-10 nm.