Usnea longissima has long been used as traditional medicine in China, India, Turkey, Canada and Europe. This lichen can produce a diverse array of bioactive secondary metabolites, many of which are unique to the organism. Their potential applications, however, are limited by their finite sources and the slow-growing nature of the organism in both laboratory and environmental conditions. This thesis set out to investigate polyketide synthase genes in U. longissima, with the ultimate goal of providing a feasible way for mass production of lichen natural products to support these applications. For further study on the diversity of PKS genes that could be detected in U. longissima, several degenerate primers targeting ketoacyl synthase (KS) domains of PKS genes have been developed and tested. Using these primers, seven KS domains from U. longissima were obtained. They included non-reducing PKS type, reducing PKS type and NRP-PKS type. Based on the KS domain phylogenetic analysis NR-PKSs were hypothesized to be involved in the biosynthesis of orsellinic acid, β-orsellinic acid and methylphloracetophenone which are the mono-aromatic precursors for many lichen-coupled phenolic compounds, such as orsellinic acid or β-orcinol depsides/depsidones and usnic acid.Six novel PKS genes were obtained by screening a Fosmid genomic library of U. longissima. Sequence analysis of six PKSs showed that the domain organization of UlPKS1 (KS-AT-ACP-ACP-TE) was quite similar to that of aromatic PKSs, and phylogenetic analysisshowed that UlPKS1 belongs to non-reducing PKSs Clade I. The domain organization of UlPKS2 was KS-AT-ACP-ACP-EST, and UlPKS2 was placed into non-reducing PKSs Clade III. The UlPKS3 domain was organized into KS-AT-ER-KR-ACP, UlPKS3 and Alternaria. solani PKSN produced independent clade that belongs to the fungal reducing PKSs. The domain organization of UlPKS4 was KS-AT-ACP-Met-EST; UlPKS4 was placed in non-reducing PKSs Clade III. According to the domain organization and phylogenetic analysis, it is possible that UlPKS4 was involved in β-orsellinic acid or methylphloracetophenone biosynthesis. The domain organization of UlPKS5 was KS-AT-ACP. UlPKS5 was among non-reducing PKSs Clade II which can produce octaketide. The domain organization of UlPKS6 was KS-AT-ACP-TE. UlPKS6 was among non-reducing PKSs Clade V which can produce tetraketide. These works provide important information on mass production of lichen natural products. For mass production of lichen polyketide, pNQ-argB-UlPKS1 and A. nidulans heterologous expression system was also constructed. Six transformants including full UlPKS1 were confirmed by the LA-PCR. Southern blot revealed that at least one copy of UlPKS1existed in transformants. TLC test showed two chemical compound bands produced by transformants. UlPKS1 from U. longissima was successfully expressed in A. nidulans, suggesting the constructed host-vector system can be a useful tool for mass production of lichen polyketide.