It is generally accepted that the origin of the structural features of low density polyethylene (branching, unsaturation, molar mass and its distribution) can be explained by various isomerisation and decomposition reactions of the macroradicals and by various chain transfer processes. On the other hand, it is known that ethylene molecules under high pressure are organised in various supermolecular particles. There are several phases in compressed ethylene (a, 0 and y) depending on the pressure and temperature. The purpose of the present work was to determine the effects of the phase state of ethylene on the structure and properties of polyethylene. The authors have compiled published data about the effects of the synthesis pressure and temperature on the structure and properties of polyethylene. The entropy of ethylene under sythesis conditions for those published experiments was determined from the available information on pressure and temperature. The effects of ethylene entropy on the number and type of short chain branches, long chain branches, unsaturated bonds, molar mass and its distribution, chain flexibility, density and the melting point of polyethylene are demonstrated. It was found that under given ethylene entropy conditions, the same structure and properties of polyethylene are obtained.