Crystalline polyglycolic acid wherein a difference between the melting point Tm and the crystallization temperature Tc₂ is not lower than 35 C., and a difference between the crystallization temperature Tc₁ and the glass transition temperature Tg is not lower than 40 C. A production process of polyglycolic acid modified in crystallinity, comprising applying heat history to crystalline polyglycolic acid at a temperature of not lower than (the melting point Tm of the crystalline polyglycolic acid+38 C.). A polyglycolic acid composition comprising crystalline polyglycolic acid and a heat stabilizer, wherein a difference (T₂-T₁) between the temperature T₂ at 3%-weight loss on heating of the polyglycolic acid composition and the temperature T₁ at 3%-weight loss on heating of the crystalline polyglycolic acid is not lower than 5 C.