Ti 5 0 . 5 - x Ni 4 9 . 5 Zr x / 2 Hf x / 2 (x=10-20 at.%) shape memory alloys exhibit the characteristics of B2 B19' one-stage martensitic transformation. The lattice parameters of B19' martensite in these alloys are determined by XRD, SADPs of TEM and Rietveld methods. Experimental results show that lattice parameters a and c, monoclinic angle β and unit cell volume V increase, but the lattice parameter b decreases with increasing x-content in Ti 5 0 . 5 - x Ni 4 9 . 5 Zr x / 2 Hf x / 2 alloys. The results of Rietveld method indicate that the martensite of Ti 5 0 . 5 - x Ni 4 9 . 5 Zr x / 2 Hf x / 2 with x=10-20 at.% has a structure similar to that of Ti 5 0 Ni 5 0 . The parameters a, c and β of B19' martensite in Ti 5 0 . 5 - x Ni 4 9 . 5 Zr x / 2 Hf x / 2 alloys are slightly larger, but b is slightly smaller than those in Ti 5 0 . 5 - x Ni 4 9 . 5 Zr x alloys for the same x-content. The B19' martensite of Ti 3 0 . 5 Ni 4 9 . 5 Zr 1 0 Hf 1 0 alloy is a monoclinic structure with a=0.3068 nm, b=0.4062 nm, c=0.4882 nm and β=101.2 o and has a substructure of (001) M compound twin.