Provided is a lubricating oil composition including a lubricating oil additive and a lubricating oil, the lubricating oil additive having nanodiamonds, of which a surface is hydrophobically modified by a surface treatment, dispersed in a base oil. The lubricating oil composition retains dispersibility over a long period of time and thus can ensure storage stability. Machines to which the lubricating oil composition is applied may have improved abrasion resistance as well as improved fuel consumption and reduced noise, and high thermal conductivity of the lubricating oil composition may also increase cooling efficiency and the service life of machines.