The site occupancy of hydrogen atom in B2-NiAl and Cr alloys and its effect on NiAl and Cr were studied by the first-principle plane-wave pseudopotential method. For NiAl system the impurity formation energies of hydrogen are -2.365 and -2.022eV in the Al-rich octahedral site and Ni-rich octahedral site, respectively, while it is unstable in a tetrahedral site. For Cr system, the impurity formation energies of hydrogen are -2.344 and -2.605eV in the octahedral site and tetrahedral site, respectively. The most stable site of hydrogen is the Al-rich octahedral site in NiAl and the tetrahedral site in Cr. It is most probable that hydrogen mainly occupies the interstitial site in NiAl/Cr alloy. By analysis of the atomic structure, electron population, valence electron density and the density of states, the effects of hydrogen atom on atomic structure and electronic structure of NiAl and Cr alloys are discussed.