The structure of Al-Li-Mg system alloy 1420, containing a small quantity of Sc, 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 S1(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 S1(Al2MgLi) phase and also δnon-phase, investigated in  are always present whereas δ'(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.