Abstract: The upstanding part of xylem caudex of Brainia insignis (Hook.) J. Sm. was extracted by organic solvents under low temperature and mild microwave irradiation. The extract was analyzed by GC-MS, 60 peaks were separated and 55 compounds were identified, which accounted for 97.88% of total peak area. The volatile components in B. insignis were classified into four structure groups: organic acids, sesquiterpenes, alkanes and cyclosiloxanes. From structural analysis, there are close correlations among individual groups of compounds. Especially in alkane and cyclosiloxane groups, there are similar structural differences between the neighboring compounds. The cyclosiloxanes have not been reported hitherto in fern research.