Poly( 1-butene) (PB) crystallizes from the melt in a metastable modification II (mod. II) which slowly transforms into the stable modification I (mod. I). X-ray wide angle (WAXS) measurements show that in mod. I the size of the microparacrystallites (mPC’s) in chain direction, D̅ 012 , the polydispersity g D of the size distribution in this direction, the lateral size D̅ 110 and the paracrystalline g 110 -value do not change upon annealing at temperatures up to the melting point. In mod. II, however, the sizes D̅ 012 and D̅ 110 increase with rising annealing temperature T ann . At a certain T ann and beyond a sufficient annealing time t ann the size D̅ 012 shows a logarithmic increase with t ann whereas D̅ 110 stays constant. Measuring melting points T m of mod. I-samples, we found a linear relationship between T m and 1/D̅ 012 according to the Thomson equation resulting in a melting point for an infinite crystal of T m ∞ (mod. I) = 139 °C and a mean surface free energy of σ̅ e ′̅ (mod. I) = 47 ergs/cm 2 . T m versus 1/D̅ 012 for mod. II is linear only for high D̅ 012 -values yielding Tm ∞ (mod. II) = 130 °C and σ̅ e ′̅ (mod. II) = 29 ergs/cm 2 . However, a partially molten and afterwards quenched sample of mod. I with small mPC’s shows a mod. II-peak which fits the straight line extrapolated from the large D̅ 012 -values. The DTA curves of mod. I-samples shift to higher temperatures and narrow after annealing although the crystallite sizes and size distributions remain as well as the paracrystalline distortions the same. X-ray and DTA measurements eliminate therefore surface premelting and selective melting of thinner and more distorted lamellae in mod. I. Upon annealing this modification, σ̅ e ′̅ decreases from 47 ergs/cm 2 to 15 ergs/cm 2 and the distribution of σ e ′ narrows. The latter determines predominantly the shape of the DTA curve. The Thomson equation therefore, applied to different samples links only the average crystallite size and the mean surface free energy with the melting point. In mod. I partial melting occurs independent of D̅ 012 and starts mainly at those mPC’s which have exposed surfaces with high σ e ′. At the beginning only single mPC’s or single lamellae melt, but no bundles of lamellae. The logarithmic increase of D̅ 012 in mod. II with t ann can be explained according to Hosemann’s model of “lateral melting” also by a partial melting of mPC’s with unprotected lateral surfaces and by a consecutive solid state diffusion of their chainsegments into the two mPC’s adjacent in chain direction, increasing the averaged sizes of the long period and the lamellae thickness.