It was already reported that the coercivity of the Co-Ni alloy thin film deposited at Ar plus N2sputtering atmosphere increases with the increase of the N2partial pressure after heat treatment in vacuum   . The change of crystalline grain size, crystalline structure and magnetic domain patterns of the film for the varied N2partial pressure were investigated by transmission electron microscopy technique. It was found that after 340°C heat treatment, crystalline structure transforms to hcp single phase and grains are completely separated when the Co-15 Ni (at%) films were sputtered at N2partial pressure greater than 75% in total pressure of 8m Torr. Foucau-lt mode and defocused mode imaging of magnetic domain patterns showed the decrease of domain width in accordance with the increase of N2partial pressure. The coercivity increase with N2partial pressure seems to be closely related to the formation of Co-Ni hcp structure which has large uniaxial magnetic anisotropy constants and to the separation of the grains and possibly to the crystalline defects in the grains.