PURPOSE: To provide a production process for making unvulcanized rubber composition which can have both characteristics of the original rubber before the formulation, even when rubbers having different properties is mixed and kneaded together.

CONSTITUTION: A starting rubber A is kneaded with a variety of additives and the unvulcanized rubber is heat-treated to prepare a half-vulcanized formulated rubber A'. The half-vulcanized rubber A' is kneaded with another starting rubber B together with additives for the rubber B to give the objective unvulcanized mixed rubber.