A process for production of a modified carbon black, at a cheaper cost, for rubber reinforcement having the superior tan .delta. temperature dependency of silica and an excellent abrasion resistance and further having no problems arising due to a low electrical conductivity is provided.In the process for production of a modified carbon black for rubber reinforcement wherein, in the step of granulating the carbon black, a water-dispersed silica is added to the carbon black, the granulating is performed by a granulator and a process of production of a rubber composition containing a surface-treated carbon black for rubber reinforcement comprising coagulating, with a coagulating agent, a mixture of (a) 100 parts by weight, as a solid content, of a diene rubber component and (b) 10 to 250 parts by weight, as a solid content, of a slurry containing a carbon black for rubber reinforcement or (b') 10 to 250 parts by weight of a modified carbon black produced by the above method.