One new pentenoic acid derivative, named 1,1'-dioxine-2,2'-dipropionic acid (1) and a new natural product, named 2-methylacetate-3,5,6-trimethylpyrazine (2), along with six known compounds (3-8), were obtained from the Cladosporium sp. JS1-2, an endophytic fungus isolated from the mangrove Ceriops tagal collected in South China Sea. Their structures were elucidated by detailed analysis of comprehensive spectroscopic data, and the structure of 1 was further determined by X-ray diffraction analyses.