In this study, flame retardancy properties of huntite/hydromagnesite mineral in plastic compounds were investigated. Prior to production of composite materials, huntite/hydromagnesite mineral was ground at particle sizes of 10 μm, 1 μm and 0.1 μm. The ground minerals with different particle size and content levels were added to the plastic compounds to produce composite materials. After fabrication of hydromagnesite/huntite reinforced plastic composite samples, they were characterized by using DTA-TG. Flame retardancy test were performed as a main objective of this research. By this way the effects of size distribution of the additives and the content levels of ceramic in the plastic compounds on flame retardancy are measured. It was concluded that flame retardant properties of plastic composites were improved depending on particle size of hydromagnesite/huntite minerals.