To monitor the reproductive patterns of endangered orangutan, Time-resolved fluorescence immuno assay (TR-FIA) were used to analyze metabolites of sex hormones in feces. Orangutan had long-term interbirth intervals (amenorrhea) during lactation period in which the secretion of sex hormones was repressed. The concentration of progesterone in the serum of pregnant orangutan was 30fold higher than that in non-pregnant orangutan. However, the concentration of hCG during pregnant period was different from the result of other primates. The present study suggested that age is not the important factor in determining the reproduction capability, because it is rather greatly influenced by the combination of various factors. Tracing metabolites of sex hormones in orangutan feces seems to be provide the effective solution for the infertility in orangutan. This study result shows the basic data in operating conservation project for endangered orangutan.