This search aims to study the effect of additive materials ( Ethyl Methyl Keton Peroxide MEKP , Cobalt Naphthenate ) at the important values of (stress-strain) curves ( stress and elongation at break ) for the preparing specimens from reinforced and nonreinforced with fiber glass unsaturated polyester resin. The result showed that the investigated parameters are clearly affected with using additive materials and the optimum concentration for non-reinforced specimens which was curing in room temperature was (1% MEKP + 1% Cobalt Naphthenate) and (0.8% MEKP + 1% Cobalt Naphthenate), and for reinforced specimens with fiber glass was:( 1% MEKP + 0.8% Cobalt Naphthenate) and ((0.2-0.4) % MEKP + 1% Cobalt Naphthenate). The results showed also that it is possible to get the better values of tensile stress at break when the specimens was curing in oven at 100 Cº and treated at this temperature for 15 min.