Dense nanocrystalline barium strontium titanate Ba0.6Sr0.4TiO3 (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950 degrees C and 1050 degrees C starting from nonagglomerated nanopowders (similar to 20 nm). The powders were synthesized by a modified "Organosol" process. X-ray diffraction (XRD) and dielectric measurements in the temperature range 173-313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature T-C decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity e shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%-4% in the frequency range of 100 Hz-1 MHz and temperature interval 160-320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer-sized ceramics.