The objective of this comparative study is to investigate the effects of electron-beam irradiation on the mechanical and stress relaxation properties of vinyl-methyl-polysiloxane (VMQ) and phenyl-vinyl-methyl-polysiloxane (PVMQ), as well the variation of these properties with irradiation depending on the silicone and co-agent type. For this purpose, firstly a series of silicone mixtures was prepared using 2,5-bis(tert-butylperoxy) - 2,5-dimethylhexane as the peroxide and either a Type I (ZDA) or Type II co-agent (TAIC). The rheological properties and curing conditions of the prepared mixtures were investigated by rubber process analyzer. In the second step, cured elastomers were irradiated with electron beams at doses of 0-80 kGy. The percent gelation of elastomers before and after irradiation was determined by sol-gel analysis. In order to investigate the effects of irradiation on the mechanical properties and cross-link density, the stress-strain behavior was determined. The stress relaxation properties of samples were determined by temperature scanning stress relaxation technique. After the irradiation, it was found that the main effect of irradiation on the silicone elastomers is cross-linking. Irradiation increased the cross-link density for both the silicone and gel fraction up to 94.3% for PVMQ and 96.7% for VMQ. A higher cross-link density was achieved for all of the non-irradiated and irradiated PVMQ elastomers than for VMQ elastomers. The relaxation studies revealed that elastomer type and irradiation are two important parameters affecting the relaxation behaviors of silicone elastomers. Increasing the irradiation dose resulted in maintaining of mechanical properties owing to the isothermal relaxation of both VMQ and PVMQ elastomers. This improvement was higher in the VMQ elastomers. In non-isothermal or temperature scanning stress relaxation behavior, an increase in the irradiation dose resulted in an improvement in the properties of the VMQ elastomers and a decrease for the PVMQ elastomers. The reason for this reduction is thought to be some undesirable side reactions occurring with the free radicals stabilized by the phenyl groups during irradiation and that these groups accelerate the decomposition at high temperatures during the relaxation experiments.