AbstractCopolymers of ethylene with relatively small amount of nine species of α‐olefins,i.e., propylene, butene‐1, pentene‐1, hexene‐1, octene‐1, dodecene‐1, octadecene‐1, 4‐methylpentene‐1, and 4.6‐dimethylheptene‐1 were prepared with the use of ZIEGLER‐NATTA catalyst and their melt properties, crystalline behaviors, and various physical properties were investigated in relation to the side‐chain length and structure.The melt index‐intrinsic viscosity (MI‐[η]) correlation, melt tension, and melt extensibility were found to be hardly affected by the branches. On the crystalline behavior and physical properties, the effects of branching generally became more pronounced when the branch sizes became larger. It was noticeable, however, that ethyl and propyl branches exert strong effects in comparison with their relatively small sizes. It was also observed that the ethylene/octadecene‐1 copolymers have lower β‐peak heights and higher β‐peak temperatures in E″‐temperature curves than expected from their branch size, and this was considered to be due to the crystallization of the side‐chains. The properties of high pressure polyethylenes coincided, in many respects, with