Fillers are poorly soluble inorganic white color pigments, carbonates, silicates, sulfates, oxides and sulfides that can be added into the paper during the industrial paper production, or later in the form of coatings in making coated papers.Filler can infuluence on many paper properties, like paper whitness increase, basis weights and water absorption, opacity, thereby contributing to the quality of the press, which is particularly evident in thin paper, especially if they are used for duplex printing. Filler particles interfere for bonding which primarily affects the reduction of paper strenght, tensile breaking strenght, bending stifness, and tearing resistance. In papers with high fillers percentage when filler reaches 30% of the total weight of paper, the retention of the fillers is uneven, which is manifested in duality of paper.In this paper, the filler content (ash content) of the paper sample will be determined. The term ash in paper and paperboard includes the non-evaporated and incombustible inorganic ingredients that remain after incineration at temperatures 550/925 °C (depending on the used method). This paper will examine filler content in several paper samples using muffle furnace unit and later comparison of the optical properties of the paper, especially with the opacity, on the device Elrepho. It is expected that papers with higher filler content will show greater opacity, regardless of their thickness and / or weights.