The influences of the polydispersity index (M-w/M-n) on the morphology and deformation behavior of high-density polyethylene solids under uniaxial stretching were investigated using in situ Raman spectroscopy. An increase in M-w/M-n led to a larger fraction of tie molecules connecting several lamellar crystals, whereas no obvious influence on the crystalline structures was observed. Although the stress-strain curves essentially remained unchanged up to the neck propagation region irrespective of M-w/M-n, the strain-hardening modulus was higher for higher M-w/M-n samples. These results indicate that the mechanical response of the network structure appears in the strain-hardening region because of the tie molecules connecting several lamellar crystals fragmented at the yield point. In addition, the molecular orientation of the crystalline chains into the stretching direction and the formation of long consecutive trans chains were enhanced for higher M-w/M-n samples.