Abstract: This investigation was carried out to justify the relationship between harvesting ages of santol fruit, browning reaction and other concerning data. Growers of santol plantations in Thailand have always accustomed to problems on inconsistency in qualities of santol fruits var. Pui Fai due to inappropriate harvesting index. Thus these encourage to decrease marketable qualities of fruits and short shelf-life particularly browning discoloration. In order to investigate adequate information for growers of the santol orchard plants, it is important to carry out experiments on Polyphenol Oxidase (PPO) activity and changes of tissue structure by separating fruit into three parts: peel, flesh and seed of santol fruit var. Pui Fai harvested at four stages: 100, 115, 130 and 145 Days After Full bloom (DAF). A factorial in randomized complete block design with five replications was used. In each replication ten fruits were used. This investigation was carried out during the period of October 2006 to March 2007 at The Department of Agricultural Technology, Mahasarakham University, Northeast Thailand. The results showed that PPO activity which related to browning reaction changed with harvesting stage. The highest PPO activities were obtained and highly significant increased in mature fruit at 130 and 145 DAF. When the fruits were ripening at 145 DAF, PPO activities from peel, flesh and seed had no significant differences and high levels by 160.76-184.44, 158.88-180.76 and 154.52-181.08 U mg-1 fresh weight, respectively. While cross-section study in peel and flesh of santol fruit var. Pui Fai at different ages indicated that immature fruits (100 and 115 DAF) showed small cells arranged one to two layers in epidermis. Trichome like-hair also appeared on this layer. When santol fruits grew through 145 DAF, epidermis converted to periderm and trichome disappeared. In addition, parenchyma cells in flesh accumulated several substances in form of druse with the fruit age increment. These high PPO activities and fruit tissue conversions of peel and flesh in mature fruit may led to be more sensitive to browning reaction.