PURPOSE: To sterilize a fermentation mediun effectively without influencing the purity of a product, by adding a monocarboxylic acid to the fermentation medium which is required for perparing a long-chain dicarboxylic acid by fermentation using a straight-chain saturated hydrocarbon as a raw material.

CONSTITUTION: One or more 8W14C monocarboxylic acid is added to a fermentation medium for 11W18C stragiht-chain saturated hydrocarbon, the medium is heated to 40W80°C and the medium for preparing the long-chain dicarboxylic acid is sterilized. An amount of the monocarboxylic acid is 200W1,000mg based on 1 little medium. In adding the 8W14C monocarboxylic acid, it is added to the medium put in the fermentation tank or can be previously added to a raw material, nutrient source, water, or thier blend. A time required for sterilization can be shortened to about one-third compared with the existing technique to improve the production efficiency extremely.