To obtain a rubber composition having a short vulcanization time without detriment to aging resistance by mixing a rubber component with a carboxylic organic compound and oxazoline and/or an oxazine compound.

This composition comprises at least one rubber selected from among natural rubbers and synthetic rubbers, 1-100 pts.wt., per 100 pts.wt. rubber, organic carboxylic compound and 0.01-10 pts.wt. oxazoline and/or oxazine compound. The organic carboxylic compounds used are desirably rosins such as gum rosin, wood rosin, tall oil resin, polymerized rosin, disproportionated rosin, hydrogenated rosin, maleinized rosin and aldehyde-modified rosin, because they can give rubber compositions capable of giving vulcanizates improved in tensile strength without marked detriment to hardness.