The reaction path and the mechanism of hydrothermal decomposition of glycylglycine (Gly-Gly) and some other dipeptides in the absence of any catalysts were investigated. Experiments were conducted at the temperature range of 240-300 degrees C and a reaction time up to 200 s. An interesting reaction phenomenon relating to the production of amino acids from hydrolysis of proteins in high-temperature water was observed. Other than hydrolysis of Gly-Gly to produce glycine (Gly), cyclodehydration also took place, producing cyclo(Gly-Gly). The product of Gly from cyclo(Gly-Gly) was also verified. The same reactions were also observed in the case of other dipeptides such as glycylleucine, glycylalanine, and glycylserine. The reaction pathway of the hydrothermal reaction of dipeptides was speculated based on the experimental results to consist of consecutive reversible reactions involving cyclodipeptides, linear dipeptides, and amino acids.