The influence of the weight-average molecular weight (M-w) on the structure-properties relationship was investigated by preparing high-density polyethylenes (HDPEs) of various M-w but constant polydispersity index (M-w/M-n) values. The thickness of the lamellar crystalline layer increased with increasing M-w, whereas the amorphous characteristics, such as the amorphous layer thickness and tie-molecule fraction, remained unchanged. The microscopic deformation of these HDPEs was investigated by in situ Raman spectroscopy. The orientation parameter < P-2 > showed a power law to M-w with an exponent of -3/4 in the strain-hardening region. The M-w dependence of the molecular orientation was interpreted as the realignment of the lamellar cluster units, the size of which was determined by the end-to-end distance of the polymer chains in their melt state. It was found that higher stretching stress is applied to the crystalline chains for HDPEs of lower M-w implying that a larger amount of taut-tie chains is formed.