In this study, the residual stresses in high-density polyethylene (HDPE) were measured using an x-ray stress measurement technique. There have been few reports published of residual stress investigations in polymeric materials by x-ray stress measurements based on conventional x-ray reflection methods. There are two problems associated with this measurement. Firstly, the diffraction peaks of the polymer in the low 2q angle region and therefore the measurement accuracy for strains reduces. Secondly, the low 2q angle region makes it extremely difficult to use the sin2ψ method. In the present study we tried to use a transmission method for measuring the residual stress in HDPE sample to resolve these problems. The HDPE sample is shaped into thin sheets which have the three kinds of crystallinity degrees. The measured data is fitted with a good linear regression line in a d-sin2ψ diagram and gradient of the regression line corresponded to the applied stress. On the other hand, the results of the residual stress measurement are deeply associated with degrees of the crystallinity in the HDPE material. The quantitative estimation of crystallinity degrees in the HDPE material was accomplished by a sink-float method. The residual stress distributions were discussed between micro-residual stresses in the crystal phase of HDPE with the amorphous phase of it. Finally, x-ray elastic constant (XEC) of HDPE was estimated and the Initial residual stresses were also measured by use of this XEC parameter.