The degree of supercooling (ΔT0) dependence of lamellar thickening growth rate (U) of an isolated extended chain single crystal (ECSC) of polyethylene is studied. The experimental formula, U = C exp(-D/ΔT0), where C = 130 nm/s and D = 20.0 K is obtained for the first time. The formula is the same as that of lateral growth rate (V). The reason why U and V obey the same formula is well explained by a model named `sliding diffusion model of the lamellar thickening growth'. The model proposed that the lamellar thickening growth is controlled by both chain sliding diffusion within the ECSC and the nucleation on the side surface. The observed fact that the U increases with increase of ΔT0 is opposite to the well known fact that lamellar thickening rate W decreases with increase of ΔT0. This significant difference was well explained by the difference between the `primary crystallization' and the `secondary crystallization', which is a kind of `Ostwald's ripening process'. The origin of the `tapered shape' is well explained by coupling of lamellar thickening and lateral growths.