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The precipitates of L21-type Ni2AlHf phase in B2-ordered NiAl system has been observed by using transmission electron microscope (TEM). The hardness of as-quenched NiAl-Hf alloys is high due to the larger strengthening. However, age hardening of this alloy is not main effect to increase hardness compared to the large microstructural variations during aging. At the beginning of aging, the L21-type Ni2AlHf precipitates keep a lattice coherency with the NiAl matrix. The orientation relationship between the Ni2AlHf precipitate and the NiAl matrix is <100>Ni2AlHf//<100>NiAl, {001}Ni2AlHf//{001}NiAl. By aging treatment for long time Ni2AlHf precipitates lost their coherency and change their morphology to the spherical ones surrounded by misfit dislocations. The orientation relationship between the NiAl matrix and the Ni2AlHf precipitates, however, has been kept even after longer aging time. The lattice misfit between the Ni2AlHf precipitate and the NiAl matrix has been calculated by the selected electron diffraction patterns, and the spacings of misfit dislocations is about 4.5% at 1173 K.