A simple, accurate and rapid colorimetry method for the determination of the cordycepic acid content in fermented mycelia of cordyceps brasiliensis henn was established. The results showed that characteristic absorption peak of cordycepic acid, mannitol, was found at 412 nm. Within a linear range of 10~50 mg/L, the regression equation was: Y=0.0102X-0.0029, R2=0.9989. The precision of the instrument was satisfactory with the RSD of 0.35% (n=6). The average recovery and RSD (n=5) were 100.30% and 1.44%, respectively. Among the tested sugar, D-fructose showed high effect on the test results. No D-fructose was found in fermented mycelia of cordyceps brasiliensis, which proved the colorimetric method was suitable for the determination of cordvcepic acid content in fermented mycelia of cordyceps brasiliensis henn. The mannitol content in fermented mycelia of brasiliensis was found to be 8.45% with the RSD value of 0.34%.