In situ Raman spectroscopy is applied to high-density polyethylene to elucidate the microscopic deformation mechanism during uniaxial stretching. The peak shifts of the C-C stretching modes of the amorphous chains show remarkable red shifts in the elastic region, where as the red shift of the C-C stretching vibration along the crystalline chain begins after the first yield point, suggesting that the stretching stress is concentrated on the amorphous chains before the first yield point.