Effects of CaO and SiO₂ addition on microstructure and intrinsic coercivity (iHc) of sintered Sr-ferrite were investigated. The amount of CaO and SiO₂ additives had direct effects upon microstructure and iHc. In CaO addition, remarkable grain growth parallel to the c-plane of Sr-ferrite crystals was observed, resulting in a decrease in iHc below 80kA⋅m^-1. On the other hand, simultaneous addition of CaO and SiO₂ suppressed the grain growth of Sr-ferrite crystals and shifted the grain size distribution to smaller values. As the amount of SiO₂ additive increased, iHc increased up to 250kA⋅m^-1 in spite of the further densification with raising sintering temperature. In this case, sintered Sr-ferrite had a homogeneous microstructure consisting of nearly isodimensional crystals in the directions parallel to the c-plane and c-axis. Therefore, the increase in iHc of sintered Sr-ferrite with suitable amounts of CaO and SiO₂ can be attributed to the decrease in average size of crystals having nearly spherical geometry, which enhances shape magnetoanisotropy of Sr-ferrite crystals.