Abstract: The resin acid fractions of Oleoresin of Pinus mossoniana consist of pimaric acid, sandaracopimaric acid, isopimeric acid, levcpimaric acid, plustric acid, abietic acid, neoabietic acid and dehydroabietic acid. During processing of the Oleoresin, the acids of pimaric acid type do not change verg much, while the acids of abietic type undergo a lot of changes. When the gum rosin is processed at temperatures between 140℃ and 200℃. there is a gradual decrease in levopimaric acid, while the plustric, neoabietic and abietic acids continue to increase. In the cooling process of rosin, the acids of abietic acid type continue to isomerize, and plustric acid and neoabietic acid decrease gradually, with the result that the content of abietic acid increases steadily from beginning fo. end.This shows the objective law cf isomerization of resin acids for the oleoresin of Pinus mossoniana under the existing processing conditions. According to the law of isomerization of resin acids, wa can make use of the relationships between the resin acid composition, the optical rotation and the tendency to crystallize. JHence, rosin of tine quality, i. e, rosin with optical rotation within suitable range and with a lower tendency of crystallization can be obtained by controlling distillation (its temperature and time) and cooling process conditibns. The optical rotation of rosin serves as an indicator of its tendency to crystallize. Fox example, when the ratio between the plustric acid and abietic acid contents of the rosin produced in Youxi, Fujian is 1:1.5~l.8, the optical rotation range will be from +4° to +12°, and the tendency to crystallize near 0%. Rosins from; any place can have a best range of optical rotation, which will result in the minimum tendency to crystallize.To prevent rosin crystallization, except for controlling the main internal factors of resin acid composition, attention must be paid to the external factors, such as temperature of the environment, crystalline seed, moisture content, addition of hot rosin, shaking, etc., any of which can cause crystallization of rosin.