Magnetic carbon black, a process for preparing the same, and a method of modifying magnetic carbon black are provided. The magnetic carbon black is prepared by process A in which when a hydrocarbon is supplied into a high temperature combustion gas stream formed by combustion of a fuel to prepare carbon black, an organic transition metal compound is supplied together with the hydrocarbon and, at the same time, an oxygen gas is supplied from the circumference of the hydrocarbon, process B in which a transition metal compound is supplied together with a hydrogen gas on the downstream side of the supply position of the hydrocarbon, and process C in which a hydrocarbon is supplied into a high temperature combustion gas stream on both the upstream side and the downstream side and a transition metal compound is supplied from a position between the supply positions of the hydrocarbon. The average particle diameter of the magnetic fine particle and the average particle diameter of carbon black in the magnetic carbon black prepared by process A are 30 nm or more and 20 nm or less, respectively. The magnetic characteristics of the magnetic carbon black can be enhanced by heating the magnetic carbon black in a non-oxidizing atmosphere at 400 to 900° C.