Water near the critical temperature has been focused as a novel reaction medium. We observed that nylon 6 can be easily depolymerized in sub- or supercritical water to produce E-caprolactam in our previous work. This indicates that the cyclodehydration reaction proceeds in water near the critical temperature. In this work, we investigated synthesis of 2-aminocaprolactam from L-lysine by cyclodehydration in subcritical water. The reaction was carried out in a batch reactor (5.4 ml in volume) in a temperature range from 573 to 633 K. The reaction time was changed from 5 to 60 min to obtain kinetic data of the reaction. The reaction products were analysed by using LC-MS and HPLC (mobile phase: 0.01 N NaOH solution). As the reaction time increased, the reaction product coloured yellow more deeply. At higher temperature, an yellow or green coloured oil phase was observed. In the products, 2-aminocaprolactam was identified by LC-MS. Lysine was cyclodehydralyzed to 2-aminocaprolactam and then further reacted to by-products in subcritical water. Lysine was completely reacted in 30 min at 633 K. The highest yield of 2-aminocaprolactam was 51% in 20 min at 603 K. For a longer reaction, the yield of 2-aminocaprolactam decreased due to secondary reaction. Subcritical water was found to be an excellent reaction medium for cyclodehydration.