The monosacchride composition of polysacchrides in Dendrobium officinal of different germplasms, physiological ages and closely related species were determined by pre-column derivatization HPLC. The results showed that the absolute and relative volumes of all monosacchrides were significantly different between D. officinale and its closely related species, different germplasms and physiological ages of D. officinale. Absolute peak areas of mannose ranged from 0.854 x 10(7) to 10.340 x 10(7) in closely related species of D. officinale, ranged from 1.467 x 10(7) to 8.475 x 10(7) in different germplasms of D. officinale and were 4.411 x 10(7) (2.577 x 10(7)-6.516 x 10(7)), 5.528 x 10(7) (3.179 x 10(7)-8.475 x 10(7)) and 3.601 x 10(7) (1.467 x 10(7)-5.888 x 10(7)), respectively, in one to three physiological ages of D. officinale. The ratio of mannose to glucose peak areas (relative peak area) ranged from 0.976 to 16.599 in closely related species of D. officinale and from 2.679 to 7.831 in different germplasms of D. officinale. Only the relative peak areas of D. pendulum and D. primulinum were in the range of different germplasms of D. officinale in all tested samples. The results revealed the variation of monosacchride composition of polysacchrides in D. officinale. Monosacchride composition of D. officinale could be altered by breeding new varieties and controlling harvesting season. Most adulterants of D. officinale could be ruled out according to the relative peak areas of D. officinale, providing a basis for quality control and resources training of D. officinale.