Formaldehyde is a carcinogenic compound that harmful to health and often misused as a food preservative in Indonesia. However, in most of the foods including fish, formaldehyde can be formed endogenously or naturally during storage. Recently, Moonfish (Lampris guttatus) from Indonesia has been exported. Meanwhile, during delayed transportation to importing countries, moonfish need to be kept frozen up to several months This condition leads to the formation of formaldehyde. The aim of this research is to study the pattern of endogenous formaldehyde formation in moonfish during the frozen storage period. The study was conducted by observing the formation of endogenous formaldehyde as well as the formation of DMA (dimethylamine), TVB (total volatile base), TMA (trimethylamine), and TMAO as parameters related to the formation of endogenous formaldehyde in moonfish. The parameters were observed periodically shortly after the fish died until six months in frozen storage. The results showed that the content of formaldehyde, TVB, TMA, and DMA in moonfish during six months of frozen storage increased every month. The formation of natural formaldehyde in the moonfish was inline with quality degradation. Moonfish is still suitable for consumption based on its freshness level up to one month in frozen storage, but based on the formation of formaldehyde, the level of consumption should consider the calculation of maximum daily intake based on specified reference dose.