Abstract <p>The influence of plasticizer content in the epoxy binder, annealing, and tensioning of the fiberglass on the mechanical properties of fiberglass-reinforced composite specimens was investigated (tests according to State standard GOST 11262–2017). The stress–strain dependence was measured using the Time WDW-50E testing system. During the measurements, the tensile process of the specimens was recorded on video. Specimens were fabricated using an ED-20 epoxy resin binder with diethylene glycol plasticizer added in amounts ranging from 0 to 50 wt %. Values of Young’s modulus and tensile strength were obtained. The addition of plasticizer and annealing of the fiberglass do not lead to significant changes in the specimens’ strength and Young’s modulus; matrix cracking does not occur during tensile testing of the composite specimens when the plasticizer content exceeds 20 wt %; pre-tensioning of the reinforcing fiber increases Young’s modulus and tensile strength of the tested specimens by 10–20%. The calculated values of the elastic modulus of the matrix and fibers allow explanation of the elastic behavior of reinforced specimens of different compositions. Video recording of the tensile tests made it possible to monitor the formation of cracks in the matrix.</p>