This article provides an Optimality-Theoretic analysis of the developmental process of vowel lengthening and shortening, especially focusing on HCL and PCS I and II. In doing so, we have proposed one faithfulness constraint and two markedness constraints. The diachronic changes could be accounted for through three stages. The constraint ranking of the first stage is Ident-IO(long) >> *V[-long] sonC[ αpl]-vdO[ αpl]]σ(V), *V[+long] CC (otherwise), that of the second stage is Ident-IO(long), *V[-long] sonC[ αpl]-vdO[ αpl]]σ(V), *V[+long] CC(otherwise), and that of the last stage is *V[-long] sonC[ αpl]-vdO [αpl]]σ(V), *V[+long] CC(otherwise) >> Ident-IO(long). Additionally, this study supports the direction of diachronic sound change agreed upon, which moves from faithfulness constraints >> markedness ones to markedness constraints >> faithfulness constraint ones. That is, our analysis reflects the promotion of markedness constraints *V[-long] sonC[ αpl]-vdO[ αpl]]σ(V) and *V[+long] CC(otherwise) and the demotion of a faithfulness constraint Ident-IO(long).