The effects of CaO and SiO₂ inter-additives on magnetic and physical properties of Ca-La-Co system ferrite sintered magnets were discussed. The results show that the maximum magnetization and remanence of the sintered magnets decrease with an increase in the ratio of SiO₂ to CaO inter-additives, because non-magnetic materials α-Fe₂O₃ are formed with an increase in the ratio. On the other hand, the coercivity H_cJ of Ca-La-Co system sintered magnets increase with an increase in the SiO₂ additives, because the average particle size of sintered magnets becomes smaller. Whereas, H_cJ of Ca-La-Co system sintered magnets decrease with an increase in the CaO inter-addition because the crystal growth of the sintered magnets occurred due to CaO inter-additive. Optimum magnetic properties and conditions in this experiment were as follows; inter-additives: 0.3 wt.% SiO₂ and 0.5 wt.% CaO, sintering temperature: 1200°C, B_r=0.429 [T], H_cJ=278 [kA/m], (BH)_max=34.5 [kJ/m³].