Autoxidation of thujopsene was investigated using gas chromatography (GC) at 100 °C and at room temperature. High yields of mayurone were detected as the main product under both temperature conditions, but products undetectable by GC, such as various peroxides and polymer, were also produced in the autoxidation. More thujopsan-9α-one and 9α,10-epoxy-8α-thujopsanol were produced at 100 °C than at room temperature. Thujopsene and its autoxidation products (thujopsadiene, mayurone, 9α,10-epoxy-8α-thujopsanol, bis(Δ9-thujopsen-8α-yl) peroxide, and thujopsan-9α-one) were tested for antitermite activity. All autoxidation products show stronger antitermite activities than thujopsene itself. Mayurone shows the highest termiticidal activity in the samples. Antifungal activities of the samples without thujopsadiene were tested against Trametes versicolor, Lenzites betulinus, Gloeophyllum trabeum, Trichoderma virens, and Rhizopus oryzae. Antifungal activities of autoxidation products are stronger than thujopsene itself, without considering G. trabeum, and mayurone has the strongest antifungal activity of all the samples. Thus, it was demonstrated that thujopsene, a compound not biologically active against termites and fungi, can be converted to economically useful compounds with antitermite and antifungal activities by simple autoxidation.