N,N'-biphenyl-N,N'-bis-(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine (TPD) and N,N'-biphenyl-N,N'-bis-(1-naphenyl)-[1,1'-biphenyl]-4,4'-diamine (NPB) are typical hole transporting materials, which are widely used in current organic electroluminescence (EL) research. NPB is regarded as a better hole transporting material than TPD, since it has a higher glass transition temperature (T-g). In this paper, the photostability and morphological stability of TPD and NPB have been studied in film states. It is concluded that NPB thin film is a better hole transporting material for organic EL than TPD, not only because it has a higher T-g, but also because it is more photostable and has a better morphological stability in thin film state.