Some food products processed meats such as sausages and meatballs allegedly tainted by contamination of non-halal food raw materials such as rat meat. Contamination with rat meat become of choice because easily obtained. The issue of falsification of beef is not just a problem for Muslims in Indonesia, but also in Indonesian society in general, especially for health. Mice is often associated with dirty animals and can transmit a variety of diseases. GCMS method is used primarily to determine the fatty acids which are the most dominant in a sample and determine the differences in the fatty acid composition of each sample so hopefully GCMS method can be used for identification of fatty rats in order to halal authentication. The method used is non-experimental. Samples meatballs made to determine the ability of GCMS in detecting the contamination of rat meat in processed meat products. The results showed, the general compounds on the cow oil and mice oil was same. Only arachidic acid, arachidonic acid and eicosadienoic acid showing in the the mice oil doesn’t appear on the cow oil. Compounds that emerged with the highest peak on the two sample is methyl trans 9 octadecenoic with retention time 40.800 minutes on the cow oil and 41.166 minutes on the oil, the compound methyl trans 9 oktadekenoat on mice oil has an area larger than the cow oil. The meatballs made from beef and mice were indistinguishable specifically because of the fatty acid content of fatty acids that exist only on mice which oils arachidic acid, arachidonic acid and eicosadienoic acid was not detected in GCMS instrument.