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As a biodegradable and biocompatible polymer material, polyglycolic acid is attracting more and more attention in the field of polymeric and biomedical materials. Consequently, the study of the transformation between different crystal forms of glycolide, the monomer of polyglycolic acid, is of great importance, because crystal forms of glycolide determine the stabilities and properties of polyglycolic acid products polymerized under different conditions in industry. Herein, this article reports the synthesis and crystallization of α‐ and β‐glycolide controlled by the type of organic solvents and/or temperatures, during which an intermediate crystal form of β′‐glycolide is newly observed. The transformation between α‐, β‐, and β′‐glycolide is investigated in detail, and the stabilities of different crystal forms are illustrated via the controlled experiments under different relative humidity (RH) as well as the theoretical calculations, which should provide fundamental concepts and guidance for the industrial production of glycolide and optimization of polyglycolic acid products. [ABSTRACT FROM AUTHOR]