We investigated the relationship between the shape of clusters of Co crystallites, obtained by transmission electron microscopy (TEM), and the magnetic anisotropy field of grains (Hkgrain) of Co-CoO obliquely evaporated magnetic tape. Comparison of the former with the latter revealed that the Hkgrain of Co-CoO obliquely evaporated magnetic tape closely correlates with the aspect ratio (N) of clusters of Co crystallites. The magnitude of Hkgrain is defined as that of a magnetic field where the rotational hysteresis loss of the torque curve is zero. We investigated the relationship between the Hkgrain and saturation magnetization Ms, using samples prepared under various conditions. The results showed that Hkgrain becomes asymptotic to the value HkCo + Ms/2, determined by the maximum value of the shape anisotropy Ms/2 in the region of low Ms. The value of Hkgrain calculated by using a value of N obtained from TEM images was found to be close to the value of Hkgrain obtained by magnetic torque measurement, and this tendency was confirmed qualitatively.