In this study, a useful and simple synthesis method of ABA-type olefin triblock copolymers, (e.g., PE-b-poly(ethylene-co-propylene)-b-PE) has been explained. The performance of the so-called "coordinative chain transfer polymerization" (CCTP) by sequential feed of ethylene and ethylene/propylene mixed gas generated a Zn-bound diblock copolymer (i.e., (PE-b-poly(ethylene-co-propylene)yl)2Zn). Treatment of the Zn-bound diblock copolymer with lauroyl peroxide (CH3(CH2)10C(O)O-OC(O)(CH2)10CH3) resulted in a C(sp3)-C(sp3) coupling reaction between the two diblock chains bound on the zinc site and an ABA-type olefin triblock copolymer, PE-b-poly(ethylene-co-propylene)-b-PE, was formed. When lauroyl peroxide was treated, the number average molecular weight increased by 1.5–1.7 times. The occurrence of the coupling reaction was confirmed using a model compound, Dioctylzinc as CTA(Chain Transfer Agent) for CCTP. The ABA-type triblock copolymer exhibited thermoplastic elastomeric properties and dramatically improved mechanical properties (twice the tensile strength and ten-fold increase in the elongation at break) as compared to the diblock congener.