The structure of Al-Li-Mg system alloy 1420, containing a small quantity of Se, Zr, Ti was investigated in cast, homogenized, hot-pressed, quenched and aged conditions, using the methods of optical metallography, transmission electron microscopy and X-ray examination. An existence of areas, having fine grains (20-30 nm)- "Ultrafine Grain areas" (UFGA) was observed in all the investigated conditions. UFGA are located on the boundaries, sub boundaries and S-1(Al2MgLi) phase particles. UFGA can also form near the particles of crystallization origin. These areas have a complex phase composition. Inside the UFGA the particles of S-1(Al2MgLi) phase and also delta(non)-phase, investigated in [1] are always present whereas delta'(Al3Li) precipitations are absent. These areas are formed during crystallization and hot deformation. Their composition changes during the treatment. The nature of these changes is considered.