Organic PCM stands for an alternative to the enhancement of thermal inertia inside buildings. In contrast, its fire reaction is an inconvenient for being used in thermal energy storage. This study focuses on evaluating fire behaviour of paraffin and fatty acid mixture ( capric acid and palmitic acid) and improving its flammability properties by the addition of flame retardants. In order to compare the flammability of PCM, the heat release rate was measured in a pyrolysis combustion flow calorimeter (PCFC). On the other hand, severa! percentages of flame retardants such as ammonium polyphosphate, hydromagnesite and magnesium hydroxide were added to organic PCM. The flame retardants influence into paraffin and fatty acid mixture was tested by the dripping test. As a result, the fire reaction of fatty acid mixtures was enhanced by the addition of 50% in weight of endothermic flame retardants systems. No improvement was observed in paraffin RT-21. The results obtained in the PCFC indicate that in case of fire paraffin would contribute more than the fatty acid mixture to fire propagation.