As the simplest and common model accounting for the mechanism of the cold-drawing of crystalline high polymeric substances, the usual theory for the rubber elasticity of vulcanized rubber was applied. The theoretical relationships between tensile strength or elongation at break and drawing ratio of drawn fibers were introduced. For example, tensile strength increases proportionally and elongation at break decreases inversely to drawing ratio. These theoretical results were in relatively good agreement with the experimental ones obtained from the drawn fibers of Phillips polyethylene. The increase of tensile strength by heat-treatment of drawn fibers seems to be attributed to the crystallization of oriented molecular chains. This fact was elucidated from the measurement of density change of both drawn and heat-treated fibers.