The transition temperature of high-density polyethylene (PE), low-density PE, and hydrogenated poly(butadiene), which was detected by the spin-labeled ESR method, was evaluated. The detected transition corresponded to the β transition of semicrystalline polymer PE. The transition temperature was found to depend on the crystallinity, the molecular weight of the sample, and labeled site. The β relaxation around chain-end segments was observed by the selective spin-labeling method. The mobility of the end segment as well as the inner segments was revealed to depend on the molecular weight (M-n) of the sample. The motional length scale (cooperative segment length) ξ of amorphous PE at T-g was estimated to be around 2 nm, which was the same order as the reported ξ for various polymers and the Kuhn length of PE.