A study has been carried out of the physical and mechanical properties and crystallization behavior of poly-3-methyl-1-butene (poly-3-MB). The melt behavior of poly-3-MB was examined by means of the depolarized light intensity (DLI) technique. These birefringence studies indicated the onset of motion in the crystalline phase of poly-3-MB at about 100°C. (over 200°C. below the melting point). These studies also showed that limited molecular motion may occur at room temperature, i.e., about 25°C. below the glass transition temperature of poly-3-MB.