Due to the low toxicity, excellent thermal resistance, optical performances and dielectric properties, cyclic olefin copolymers (COCs) could be widely applied in manufacturing of food and medicine package, optical components, electronic device, and so forth. However, most of the commoditized COCs possessing glass transition temperatures (T_gs) higher than 150°C always show non-ignorable mechanical brittleness. Herein, a bulky cyclic olefin, 5-norboenene-2,3-dicarboxylicacid-adamantan-1-yl ester (NBDAE), was synthesized by one step reaction in high yields and subsequently employed to copolymerize with cis-cyclooctene (COE), affording a series of COCs bearing bulky adamantane side group. Due to the incorporation of the bulky norbornene derivative (NBDAE), the obtained COCs exhibited high T_gs (up to 216°C) and transparency (higher than 90%), even at relatively low NBDAE incorporations. As expected, the NBDAE-COE copolymers also showed improved mechanical properties compared to commercialized Zeonex® 330R (Stress at break: 39.1 MPa, strain at braek: 2.3%). Most of the synthesized COCs showed thermal decomposition temperatures (T_ds) higher than 300°C and did not decompose under maintaining at 250°C for 1 h. Considering the performances mentioned above, the prepared COCs would show more greatly potential application in various fields.