This study was focused on the volatile nanoparticles consisting of hydrocarbons under idling and deceleration conditions. Diesel nanoparticles were collected on aluminum substrates using an impactor (MOUDI and Nano-MOUDI) and were analyzed with a gas chromatography-mass spectrometry (GC/MS) and a time-of-flight secondary ion mass spectrometry (TOF-SIMS). Main components of the volatile nanoparticles under idling and deceleration conditions were some combination of the lubricating oil and the fuel heavy hydrocarbons (> C19). Oxygenated fuel hydrocarbons and > C35 lubricating oil hydrocarbons which had lower volatility than the main components had were detected in the volatile nanoparticles under idling and deceleration conditions respectively. We could simulate the volatile nanoparticles were formed via homogeneous nucleation of these lower volatile molecules, considering the existence of the lower volatile molecules in engine-out exhaust. It was concluded that the lower volatile molecules (oxygenated fuel hydrocarbons and > C35 lubricating oil hydrocarbons) were nucleation materials and the formation of the volatile nanoparticles consisting of hydrocarbons was dominated by homogeneous nucleation.