Abstract: Particle length and relative crystallinity of eucalyptus powder obtained by common milling and liquid nitrogen milling were measured by light microscope and XRD. The results showed that the particle length of eucalyptus powder was decreased with increasing of milling time, and the average length of eucalyptus powder with liquid nitrogen milling was smaller. In addition, coefficient variation of particle length in eucalyptus powder was large under the two different milling conditions, and the coefficient variation of powder obtained by common milling was decreased with increasing of milling time, but there was no abvious regularity when the powder was obtained with liquid nitrogen milling. On the other hand, relative crystallinity of eucalyptus powder was decreased with extending milling time under the two different milling conditions. Moreover, relative crystallinity of eucalyptus powder with common milling was smaller in the first 30 minutes under the same milling time, but relative crystallinity of eucalyptus powder with common milling and liquid nitrogen milling were similar after 30 minutes. Particle length of eucalyptus powder could be improved extremely with liquid nitrogen milling, and relative crystallinity of it was changed very few in a certain period of some time.