We investigated the effects of finite lengths of polymer chains on the structural and mechanical properties of polyethylene (PE) fibers with atomistic molecular dynamics simulations, PE fiber models containing long but finite chains with different distributions of chain-end defects were prepared from the orthorhombic crystalline PE configuration. In our main PE fiber model, chain-end defects were uniformly spaced along the chain direction with chain ends located at boundaries of multiple crystalline regions, which is consistent with the distribution of chain ends found in semicrystalline PE. At the early stage of tensile deformations before the yield point, a mismatch of conformational preferences in the chain-end region of a chain and chain segments away from the chain-end regions in neighboring chains developed gradually. Near the yield point, this mismatch in conformational preferences triggered a yield behavior associated with chain slippage near the chain-end region and caused significant changes in the structural and mechanical properties. After the yield point, the pattern of changes in effective lattice parameters estimated with the PE fiber model with finite chains was significantly different from that estimated with the crystalline PE without chain ends but was in an excellent agreement with those observed in recent experiments on PE fibers and preoriented high-density PE films. Rotations of chains in specific directions in response to the conformational mismatch with neighboring chains caused the observed pattern of changes in lattice parameters.