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Long-chain alkanes are a type of important intermediate-volatile organic compounds (IVOCs) in the atmosphere, which contribute to a large proportion of secondary organic aerosol (SOA). However, the optical properties of SOA derived from long-chain alkanes remain poorly understood. Here, we investigate the refractive index (RI) of SOA derived from photo-oxidation of dodecane (C12), pentadecane (C15) and heptadecane (C17) under low-NO x and high-NO x conditions with the absence or presence of inorganic aerosol seeds. The RIs of these SOAs are found to be in the range of 1.33 to 1.57 at the wavelength of 532 nm. The results from mass spectroscopy indicate that both reaction mechanisms influenced by NO x level and gas-particle partitioning influenced by seeds have important impact on the chemical compositions of SOAs, which further influence the optical properties like RI. Finally, by comparing the RI values to other literature and model results, we suggest that various RIs of SOAs derived from long-chain alkanes should be applied in atmospheric and climate models. [ABSTRACT FROM AUTHOR]