Molecular distributions and stable carbon isotopic (δ13C values) compositions of dicarboxylic acids and related secondary organic aerosols (SOA) in PM2:5 aerosols collected on a day/night basis at the summit of Mt. Tai (1534 m a.s.l.) in the summer of 2016 were analyzed to investigate the sources and photochemical aging process of organic aerosols in the forested highland region of the North China Plain. The molecular distributions of dicarboxylic acids and related SOA are characterized by the dominance of oxalic acid (C2), followed by malonic (C3), succinic (C4) and azelaic (C9) acids. The concentration ratios of C2 = C4, diacid-C = OC and C2 = total diacids are larger in the daytime than in the nighttime, suggesting that the daytime aerosols are more photochemically aged than those in the nighttime due to the higher temperature and stronger solar radiation. Both ratios of C2 = C4 (R2 > 0:5) and C3 = C4 (R2 > 0:5) correlated strongly with the ambient temperatures, indicating that SOA in the mountaintop atmosphere are mainly derived from the photochemical oxidation of local emissions rather than long-range transport. The mass ratios of azelaic acid to adipic acid (C9 = C6), azelaic acid to phthalic aid (C9 = Ph) and glyoxal to methylglyoxal (Gly = mGly) and the strong linear correlations of major dicarboxylic acids and related SOA (i.e., C2, C3, C4, !C2, Pyr, Gly and mGly) with biogenic precursors (SOA tracers derived from isoprene, α/β-pinene and β-caryophyllene) further suggest that aerosols in this region are mainly originated from biogenic sources (i.e., tree emissions). C2 concentrations correlated well with aerosol pH, indicating that particle acidity favors the organic acid formation. The stable carbon isotopic compositions (δ13C) of the dicarboxylic acids are higher in the daytime than in the nighttime, with the highest value (−16:5 ± 1:9 ‰) found for C2 and the lowest value (−25:2 ± 2:7 ‰) found for C9. An increase in δ13C values of C2 along with increases in C2 = Gly and C2 = mGly ratios was observed, largely due to the isotopic fractionation effect during the precursor oxidation process.