To investigate the quality retention method of salted and semi-dried mackerel fillets, pH, volatile basic nitrogen (VBN), trimethylamine (TMA) and total bacterial counts in the fillets that were UV treated for 1, 3, and 4 hr were periodically measured during 48 days of refrigerated storage. The moisture, crude protein, crude lipid, and ash contents were 45.46 %, 18.87 %, 33.75 %, and 2.99 %, respectively. The freshness as determined by pH measurement could be maintained up to 25 days for the control and 31 days for the UV-treated fillets. VBN contents were continually increased over the storage time. The control reached the initial putrid level after 21 days while the UV-treated fillets after $25{sim}31$ days. A similar tendency was shown for the changes of trimethylamine TMA contents, although the period differed slightly and the increase of the UV treatment time could decrease the TMA contents. The changes on the total bacterial counts of the fillets were maintained under 105 CFU/g during 48days. Especially, due to the effective decrease of the microorganism count by UV treatment at the beginning of the storage, UV -treated fillets showed lower bacterial counts than control over the whole period. E. coli. and Listeria were not detected in any fillets. Combining the above data and the sensory changes, the storage period of the fillets could be prolonged from 15 days at present to 30 days by UV treatments.