In this communication we try to provide a straightforward but critical discussion on the meaning of the degree of crystallinity defined by the enthalpy of melting. We show that the widely used crystallinity obtained by Delta h(f)/Delta h(f)(0) can be called the ’equivalent weight crystallinity’, signifying the amount of perfect crystals that can be melted by the measured enthalpy of melting. The equivalent weight crystallinity does not only depend on the amount of the crystals, but also on the perfection of the crystals : a crystal with higher perfection has larger equivalent weight. The weight fraction of the chain-folded crystals is derived based on the same model and assumptions underlying the Thomson-Gibbs equation. A reason is also offered for the frequently observed difference between the crystallinity determined by the density and that obtained by the enthalpy of melting.