With hydrogenation and dehydrogenation, lattice of hydrogen absorbing alloys expands and contracts in 20 to 30 % in volume. Defects are expected to be introduced into the lattice to relax the strain energy that is generated by volume expansion and contraction. We have studied defect formation into the lattice of LaNi5 and related alloys using in-situ diffraction and positron lifetime measurements. In the lattice of LaNi52, a large amount of defects such as dislocations and vacancies was introduced at the first hydrogenation. Once defects were introduced, they were not relaxed at the working temperature of the alloys. Using diffraction method, we measured the size of crystallite before and after hydrogenation. The crystallite size did not changed with hydrogenation in all the alloy studied; LaNi5 and LaNi5-xMx (M=Al, Sn). Volume expansion and contraction can be accepted by introducing defects in the lattice without reducing the crystallite size.