This study investigates the influence of aggregates, glass fibers and a coupling agent on the compressive and flexural (three-point and four-point bending) behavior of a polyester mortar. Particle size of fine aggregates (quartz and limestone) varied from 0.1 to 5 mm (0.004 to 0.2 inch) and the glass fiber content was varied up to 6% by weight of mortar. A silane was introduced into the polyester mortar by pretreating the aggregates and the glass fibers. The mechanical properties of mortar were studied at room temperature. The test results indicate that the selection of aggregate type, size and distribution is very important. Silane treated aggregate systems showed more than 66% increase in compressive strength and 35% increase in flexural strength when compared to the untreated systems. Addition of glass fibers enhances the strength and toughness of the polyester mortar, and silane treatment of glass fibers helps to further enhance these properties. Flexural (three-point bending)-to-compressive strength ratio varied from 0.28 to 0.35 for unreinforced system and from 0.26 to 0.54 for the reinforced system. The mortar with only 14% polyester and 86% aggregates (by weight) and a coupling agent had a compressive strength of 103 MPa (15,000 psi) which is 94% of the polyester polymer strength. A stress-strain relationship is proposed to represent the complete stress-strain response under compression and flexural loading. Also, a method is proposed to quantify the failure patterns.