The present invention provides an internal olefin composition comprising a mixture of 60 to 80% by mass of an olefin having 16 carbon atoms and 40 to 20% by mass of an olefin having 18 carbon atoms wherein a content of an α-olefin in the mixture is 10% by mass or less, and a content of a branched olefin in the mixture is 10% by mass or less, which has a good biodegradability even when discharged into the environments, a less toxicity against marine organisms, etc., and a sufficient fluidity when used as a base oil for oil drilling, etc.