Beetroot peels (BR) fibers have been employed as a natural reinforcement for Unsaturated Polyester (UPE) due to their low cost, biodegradability characteristics and eco-friendliness. Beetroot peels particulates of about (180 and 75 µm) sieve siz, of different weight percentage (1, 2, 3, 4 and 5 wt.% % ) were used as fillers in a (UPE) matrix. The effect of the beetroot particle size on physical, mechanical and structural properties of the final natural composite. The mechanical properties, such as hardness and impact strength, was investigated. The results of the physical tests show that the density of (UPE/ 180 MBR) composite increases by increasing weight percentage of beetroot peels powder, but by using the beetroot peels with the particle size (75 microns) , the density of (UPE/ 75 MBR) has decreased by increasing weight percentage of beetroot peels powder. The thermal conductivity values of (UPE/ MBR) composites were increased by increasing the weight percentage of beetroot peels powder. The thermal conductivity values of (UPE/ 180 MBR) composites have higher values, by compared with the thermal conductivity values of (UPE/ 75 MBR) composites. The results showed that the best mechanical properties were get by adding (2 Wt.% ) to the (UPE)matrix using (180 MBR) and at ( 4 Wt.% ) by adding the ( 75 MBR) . The techniques (EDS) and (SEM) were used to investigate the chemical compositions and the homogeneity degree of both (UPE/ 180 MBR) and (UPE/ 75 MBR) composites, respectively.