Semicrystalline polymers are extensively used in various forms, including fibers, films, and bottles. They exhibit remarkable properties, e.g., mechanical and thermal, governed by hierarchical structures comprising 10–20-nm-thick lamellar crystals. It is commonly recognized that long polyethylene (PE) chains fold to form thin single lamellar crystals, with the molecular chains being perpendicular to the flat faces of the crystals (the chain-folding model). Chains inclining to such vertical orientation in single crystals have since been reported, along with their effects on the physical properties of PE. For bulk specimens, the chain tilt angle (φ) has been investigated only for model samples with well-annealed internal structures. However, the φ values of lamellae and their origins are controversial for briefly annealed specimens due to the disordered lamellar morphology and orientation. Herein, we report the direct determination of molecular-chain orientations in the lamellar crystals of high-density PE using a state-of-the-art electron-diffraction-based imaging technique with nanometer-scale positional resolution. Clarifying the nanoscale variation in lamellar crystals in PE can allow precise tuning of properties and expedite resource-saving material design.