The dissolution, crystallization and hydrolysis behaviors of polyamide 6(PA 6) in superheated water(140 ℃ ≤ TH≤ 200 ℃ ) are investigated. The hydrothermal processing of PA 6 can be divided into four regions:(Ⅰ) TH < 140 ℃ ,(Ⅱ) 140 ℃ ≤ TH ≤ 155 ℃ ,(Ⅲ) 155 ℃ < TH < 160 ℃ and(Ⅳ) TH ≥ 160 ℃ . Below 140 ℃ , the hydrothermal processing does not have obvious impact on PA 6. Between 140 ℃ and 155 ℃ , an annealing effect is observed that the hydrothermally treated resin shows increased melting temperature and lamellar thickness compared with the original PA 6. Between 155 ℃ and 160 ℃ , the hydrothermal processing induces both annealing and surface swelling. Above 160 ℃ , PA 6 dissolves fully in the superheated water. As PA 6 dissolves in the superheated water, hydrolysis takes place and becomes more prominent at higher temperatures and longer processing time. The hydrolysis induced molecular weight decrease fits an exponential decay. The dissolution, crystallization and hydrolysis behaviors of polyamide 6 (PA 6) in superheated water (140 ° C ≤ TH ≤ 200 ° C) are investigated. The hydrothermal processing of PA 6 can be divided into four regions: (II) 140 ° C ≤ TH ≤ 155 ° C, (Ⅲ) 155 ° C