Octamethyl-POSS-HDPE nanocomposites were prepared by the melt mixing route. The theological and viscoelastic behavior of HDPE-octamethyl-POSS nanocomposites was investigated with a strain-controlled rheometer and dynamic mechanical analysis (DMA). The theological results show that while at lower filler concentration (0.25-0.5 wt %) POSS particles act as lubricant and reduce the complex viscosity of the nanocomposites, at higher concentrations, viscosity increases. POSS remains miscible in HDPE at lower concentrations and lower temperatures used in the study; however it starts aggregating at higher concentrations and higher temperatures. The presence of POSS particles causes gelation in HDPE matrix at concentrations higher than 5 wt % possibly due to the segregated particle-particle interaction. The storage modulus shows an increase at lower POSS concentrations but decreases at higher POSS concentrations. The alpha-transition temperatures shift to higher values at low POSS concentrations and is attributed to restriction of the movement of HDPE chain segments in the intercrystalline regions due to the presence of POSS macromers around the crystallite boundaries. The effect of POSS on morphology of HDPE was studied with X-ray diffraction and DSC techniques. It was observed that while POSS does not interfere with the crystallization of HDPE, it crystallizes itself into nanocrystals.