In this article, composites with polyurea as the matrix were prepared. Fly ash (FA) is a waste product of thermal power stations generated in huge quantities and consists of hollow particles with porous shells. These particles were employed as the filler. The volume fraction and particle size of FA were varied to study their effects on the density and tensile properties of the composites. The tensile tests were performed using an Instron load frame combined with a digital camera. Scanning electron microscopy (SEM) was used to observe the fracture surfaces of the composites. Results indicated that the addition of FA linearly decreased the density of the composites. Tensile stress and elongation at break of all composites decreased with an increasing volume fraction of FA. The moduli at 100 and 300% elongation of the composites with small- or medium-sized FA particles increased up to a certain value and declined with further addition of FA. Fractographic analysis showed that large FA particles were crushed, while finer particles tended to debond.