The nanostructure of oleyl acid phosphate (OLAP) in lubricant, which is an essential factor for understanding lubrication properties, was investigated using small-angle X-ray scattering (SAXS). SAXS profiles were observed for OLAP with concentrations of 1, 3, 5, and 10 mass % in poly-alpha-olefin (PAO). The SAXS results demonstrated that OLAP forms reverse-micellar-like aggregates in PAO. This nanostructural information from SAXS will drive further development of advanced lubricants.