Nanosized Co spinel ferrite was prepared by a combustion method. The grain size of resultant ferrite can becontrolled by the reaction conditions. Magnetic investigation revealed that the products with an average grain size of 4 nm behave as superparamagnetic materials and have zero coercive force; whereas the samples with an average grain size of 85 nm (near the size of critical single domain) have the maximal coercive force. There were the negative magnetic intergrain interactions which became stronger when the grain size neared that of the single domain. Nanosized Co spinel ferrite was prepared by a combustion method. The grain size of resultant ferrite can be controlled by the reaction conditions. samples with an average grain size of 85 nm (near the size of critical single domain) have the maximal coercive force. There were the negative magnetic intergrain interactions which became stronger when the grain size neared that of the single domain.