A copolyester having a low melting point and a high crystallinity includes a first recurring unit, a second recurring unit, and a third recurring unit, which are respectively represented by Formulas (I), (II), and (III) defined herein. A molar ratio of the first recurring unit to the second recurring unit ranges from 1:0.5 to 1:3, and a molar ratio of the first recurring unit to the third recurring unit ranges from 1:1.3 to 1:8. The copolyester may have a melting point lower than 220° C. and a lattice enthalpy greater than 12 J/g. A process for producing the copolyester and a fiber made from the copolyester are also disclosed.