The Nb was the most effective alloy elements of Ti3Al alloy on RT plasticity in the late 1970’s. The effects of Nb content in as cast α2 based Ti-25at.%Al-(11-16)at.%Nb alloys made by plasma arc melting on structures and mechanical properties were studied in this paper. The Prior /α2 grain boundaries were found to increase both ductility and strength. The size of α2 plates resulted in finer α2 cleavage facet which would be fine when Nb content was increased. They would cause high fracture strain and strength in high Nb content alloys. The lattice ratio of 2 phase will be decreased when the Nb was increased. It was supposed that the Nb could substitute for the Ti so that the distance of a axis between atoms would be increased. According to Hall-Petch formula with σ=σ0+kD-1/ the grain boundary int these three alloys was calculated and knew that σ0 is 168MPa, K is 10.8MPam-1/2. The effective turn of four respects is lattice constant c> c/a> V> lattice a in terms of variance method with the deviation in these three alloys that means that lattice c is the best one and then c/a& V and finally lattice a in terms of deviation.. It was found the calculated value c/a is 0.81 meantime volume of unit has been 0.42nm3 and their standard deviation is about 0.16% and 0.32% respectively. It expresses that the later volume is not preciser than the former c/a.