Abstract: Introduction: Food safety is determined to assure that food safety hazards present in the food have been eliminated, reduced to an established acceptable level or prevented from exceeding the acceptable level. Fish and seafood suffering from certain diseases need to be treated by MG, which metabolized to LMG. Such fish are cooked before its consumption may cause health problems. Many analytical methods for determining MG and LMG residues were required expensive apparatuses, large efforts and long time to done. Objective: Establishing a rapid and accurate Thin Layer Chromatographic (TLC) method for determining MG and LMG in fish and to follow up the effects of various cooking methods on MG and LMG residues in treated fish. Methodology: Thin-layer chromatography, roasting, frying and microwaving were used in this study. Results: Limit of quantification (LOQ) for MG and LMG was 0.2 ng g–1. Standard curves were linear for MG and LMG with correlation coefficients of 0.9913. No interferences occurred for MG and LMG on the developed TLC of blank samples. Average recoveries of raw, roasted, fried and microwaving samples were given. The losses observed in LMG were 18, 22 and 30% for roasting, frying and microwaving, respectively. The relative standard deviations were fewer than 15% for malachite green and fewer than 12% for leucomalachite green. The decrease of cooked samples weights of raw muscle, during roasting, frying and microwaving was observed. Conclusion: It could be conclusion that high temperatures and cooking processes do not ensure a full elimination of both malachite green and leucomalachite green which may be present in Tilapia fish.