Formalin is used as a preservative in medical laboratories. was used in vegetables, meat and seafood to keep freshness or shelf life for longer time without signs of spoilage. Formalin is very harmful to human health that can cause headache, chest pain, nausea, diarrhea and possible death when with large doses of formalin is taken. The purpose of this study was to determine the concentration of formalin in four fish consist of mackerel (Rastrelliger brachysoma), skipjack (Euthynnus affinis), threadfin bream (Nemipterus nematophorus) and mullet (Liza vaigiensis) from three different markets include Municipal Market, Sampaothong Market and Nasoon Market in Muang district, Surat Thani province. The results showed that skipjack had the highest formalin concentration 0.932 ppm and mackerel had the lowest formalin concentration 0.287 ppm. Fishes were washed with water for first and second time that can reduce formalin 7% and 14%, respectively. The effect of cooking include boiled and fried shows a reduction of the formalin concentration 23% and 19%, respectively. Sampaothong Market was found to be most formalin treated fishes. However, the formalin concentration in all fish species and fish circumstances were still lower than the permissible level set by Department of medical sciences ( 5 ppm ). Therefore, washing 2 time and cooked by boiling can potentially reduce the formalin concentration in fish.