Having the smallest atomic radius, hydrogen can easily enter into many metals either by intentional doping or by inadvertent exposure to humid air. For Ti-Ni-based shape memory alloys, our recent studies have shown a specific effect of hydrogen in this important class of alloys, i.e., the interaction between hydrogen and twin boundaries can result in a very high damping peak in the martensitic state; thus hydrogen can be utilized to develop high damping materials. In the present study, aiming to provide a more comprehensive investigation to the role of hydrogen in Ti-Ni-based shape memory alloys, we systematically studied the hydrogen effect on martensitic transformation behavior, and twinning stress. By comparing the results from two sets of samples, one with and another without hydrogen, we obtained the following results. Firstly, hydrogen can lower the martensitic transformation temperature and reduce the transformation heat. Secondly, the twinning stress increases with hydrogen doping, and a typical yielding was found in the tensile testing for the sample containing hydrogen, which is absent in the H-free sample. This indicates that the hydrogen-twin boundary interaction also affect dc mechanical properties.