As a TCM, Hedyotis diffusa Willd. has been using to treat malignant tumors, and many studies also showed that the extracts from Hedyotis diffusa Willd. possessed evident antitumor activities. Therefore, we carried out chemical study on Hedyotis diffusa Willd. and investigated the cytotoxicity of the obtained compounds on a panel of eight tumor cell lines. As a result, four new compounds were isolated from Hedyotis diffusa Willd., including three iridoid glycosides of Shecaoiridoidside A–C (1–3) and a cerebroside of shecaocerenoside A (4). Also, six known iridoid compounds (5–10) were also obtained. The cytotoxicity of all compounds against human tumor cell lines of HL-60, HeLa, HCT15, A459, HepG2, PC-3, CNE-2, and BCG-823 were also evaluated in vitro. New compound 3 exhibited evident cytotoxicity to all tumor cell lines except the Hela, and the IC₅₀ values are from 9.6 µM to 62.2 µM, while new compound 4 showed moderate cytotoxicity to all the cell lines, and the IC₅₀ values are from 33.6 µM to 89.3 µM. By contrast, new compound 1 and known compound 9 showed moderate cytotoxicity to HCT15, A459, and HepG2 selectively. Known compound 7 also exhibited moderate cytotoxicity to HCT15 and A459 selectively.