A process for preparing modified diene polymer rubbers having an increased impact resilience and a reduced hardness at low temperatures and useful as rubber materials for automobile tires and other industries which comprises producing an alkali metal-containing conjugated diene polymer, and reacting the alkali metal-containing polymer with a modifier selected from the group consisting of a nitro compound, a phosphoryl chloride compound of the formula: ##STR1## wherein R.sup.1 to R.sup.4 are independently an alkyl group, and an aminosilane compound of the formula: ##STR2## wherein R.sup.5 to R.sup.7 are independently an alkyl group or an alkoxyl group, R.sup.8 and R.sup.9 are independently an alkyl group, and n is an integer, said alkali metal-containing diene polymer being prepared by a living anionic polymerization using an alkali metal-based catalyst or by an addition reaction of a diene polymer having conjugated diene units and an alkali metal-based catalyst in a hydrocarbon solvent. The modified diene polymers can be incorporated with usual rubber additives to provide rubber compositions for various purposes, and the cured products shown an increased impact resilience and a reduced hardness at low temperatures.