Abstract: An analysing method of trace isoalkanes and cycloalkanes in geosamples are introduced in this paper. Its process includes a separation of trace isoalkanes and cycloalkanes from the total alkane fraction, and henceafter a separate GC analysis. The method, being simple, capable of ultra micro-analysis and high efficiency of separation, provides new analytical techniques for micro-hydra carbon samples, particularly those sampled from marine carbonate rocks with low organic content and high maturity in the area of the southern China.