In the present study, the physical, chemical, thermal and mechanical properties of polyesters used in the production of composite materials were investigated by using initiators at different ratios. In scope of the study, orthophtalic and isophtalic resins were used. Methyl ethyl keton peroxide (MEKP) has been used as initiator with different ratio; 1,0%, 1,5%, 2,0% and 2,5% (v/v). Cobalt octaoate (C16H30CoO4) was added as catalyst in the rate of 1% to increase curing rate and yield of polymerization. Gelation temperature, gelation time, peak exotherm temperature and viscosity analysis were carried out to determine those physical parameters. Thermal stabilities were determined with Thermogravimetric analyzes (TGA). Fourier transform Infrared Spectroscopy (FT-IR) was used to investigate the molecular interaction in the end of the polymerization. Mechanical properties (tensile strength, elastic modulus, flexture strength and flexture modulus) were determined with the Universal Test Machine. As the result, the mechanical properties of the unsaturated polyester resin changed based on the initiator ratio. When the relation between initiator ratio and the mechanical properties is examined, mechanical strengths of the composites composed of the isophthalic resin have generally higher than the composites composed of the orthophtalic resin. Optimum initiator ratio for mechanical strength values were determined as 1% for the composites obtained from isophtalic and orthophtalic resins.