Crystallinity of slash pine (Pinus elliottii Englem.) wood was measured by X-ray diffraction (XRD) technique, and the correlation of wood crystallinity with annual ring characteristics (ring width, earlywood width, latewood width, latewood percentage and distance to the pith) and chemical composition (α-cellulose and lignin) was investigated. Wood crystallinity was significantly correlated with ring width, earlywood width, latewood width, and contents of α-cellulose and lignin. Compared with the regression model of wood crystallinity based on either annual ring characteristics or chemical composition, the data was bested fit by the regression model consisting of both annual ring characteristics and chemical composition according to the adjusted R2. These results suggest that wood crystallinity can be estimated by tree annual ring characteristics and chemical composition, and on the other hand wood crystallinity can also be an important index to evaluate the growth traits of trees and wood properties.