The present invention relates to a catalyst system for olefin polymerization and a method for olefin polymerization using the same. More particularly, the present invention relates to a Ziegler-Natta catalyst system for olefin polymerization comprising a transition metal compound as a main catalyst component, an organic metal compound as a cocatalyst component, and a cyclic organosilane compound, trioxasilocane as an external electron donor, and a method for olefin polymerization using the same. According to the present invention, a polyolefin having improved various properties such as polymerization activity, melt-flow ability, isotacticity, molecular weight distribution and apparent density can be produced.