Octamethylcyclotetrasiloxane and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane were ring opening polymerized to prepare vinyl terminated poly(dimethyl-co-methylvinyl)siloxane and the effect of the reaction conditions on the molecular weight, the reaction yield, the structure, and the composition of the copolymer was studied. When excess end capping agents are added and the reaction is carried out under a condition where its reaction is favorable, ring opening occurs rapidly and high reaction yield and low molecular weight products can be obtained. When the reaction temperature is set far from the activation temperature of the end capping agent, relatively high molecular weights can be obtained, however, the time required for ring opening increases and the reaction yield decreases significantly. At 120℃where the ring opening occurs relatively stably, minimizing the amount of end capping agent increased the molecular weight and the reaction yield simultaneously. Due to the lower reactivity of octamethyl-cyclotetrasiloxane relative to 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane, at lower copolymerization temperatures the content of dimethyl siloxane block in the copolymer is lower than the feed composition and the product is mainly random copolymers.