Lemon peel fibers with micro particle size (MLP) have been used as an eco-friendly plant reinforcement for Unsaturated Polyester (UPE) . Unsaturated polyester was loaded by Lemon peel particles of two different particle sizes (180 and 75 microns) and with different weight percentages (1 - 5 %) to fabricate yellow gelcoat with good properties. The effect of both the size and weight percentage of lemon peel fibers on physical properties (density and thermal conductivity), mechanical properties (hardness and impact strength) and structural properties such as energy Dispersive Spectroscopy (EDS) and Scanning Electron microscopy (SEM) on unsaturated polyester composites has been investigated. The results show that the density of both (UPE/ 180 MLP) and (UPE/ 75 MLP) composites increases by increasing weight percentage of Lemon peel fibers. The unsaturated reinforced by (4% wt ) of lemon peels fibers with particle size ( 75 microns) has higher thermal conductivity value (0.327 watt/ m.c ) compared with the thermal conductivity value ( 0.248Watt/ m.c) for the unsaturated polyester reinforced by (4% wt ) of lemon peels fibers with particle size (180 microns) .The (UPE/ 2% 180 MBR) and (UPE/ 4% 75 MBR) composites shows the best hardness and impact strength values. The techniques (EDS) and (SEM) were used to determine the chemical compositions and the homogeneity degree of the two unsaturated polyester composites, respectively.