Friction stir welding is a novel welding technique which has been successfully applied in structure manufacturing of aluminium alloys (including 2XXX and 7XXX). Friction stirring in aluminium alloy welds produces a combination of very fine grain size. This paper deals with the SPF response in 1420 Al-Li alloy welds. The results showed that the friction stir weld could achieve good superplasticity performance. The lateral elongation in the weld could reach 120% and the grain size still remain the same size with little change. And the drawback in the friction weld is the obstacle to fufill the good SPF ability. During the friction stir welding, some oxides or contaminates were stired into the weld in the form of black line. When the weld was experienced superplastic deformation, the crack initiated from the line and finally reach the surface and bottom of the plate. The oxide was formed mainly in the nugget of the friction weld. So the parameters and preparation before welding must be optimized so that the weld can achieve maximum superplasticity performance.