Suspended particulate materials (SPM) were investigated for their iron isotope composition so as to assess their isotope behaviors during the iron biogeochemical cycle in summer in the Aha Lake of Guizhou Province. All the samples were purified with the anion exchange chromatography (Bio_Rad AGMP_1 resin, 100～200 mesh) and the iron isotope composition was analyzed on Nu Plasma MC_ICP_MS. δ56Fe alues of SPM sampled in the lake and the rivers display statistically a negative shift compared with IRMM_014. The values of lake samples vary from －136‰ to －010‰, while those of river samples range from －088‰ to -016‰. Besides,the average iron isotope composition of aerosol sampleses+006‰±002‰.Theoplankton was also sampled and analyzed for the iron isotope composition of the whole sample including intracellular and extracellular iron, which yielded a value of +0.08‰. It is suggested that the complex biogeochemical process plays an important role in changing the δ56Fe values of SPM in the lake apart from various inputs. During stratification in summer, it is the organically bonded iron particles that make the iron isotope composition of SPM in the epilimnion light, with an average value of －0.29‰. An iron cycle is established near the redox boundary where the upwardly diffusing Fe(Ⅱ) is oxidized and the iron particles formed will continuously sink back to the reducing zone, with which the cycle is completed and the δ56Fe values of SPM reach the minima－0.88‰ (DB) and －1.36‰ (LJK), just below the redox boundary. Due to random transportation and diffusion, the profiles of δ56Fe values of the SPM near the redox regions develop to an approximate Gaussian shape. Besides, the formation of ferrous sulfides may be another reason for the light iron isotope composition of the SPM near the water-sediment interface. Moreover, there is a good negative correlation of the δ56Fe value with the Fe/Al ratio especially for DB location, which further indicates that the transformation and diffusion of reactive iron play an important role in modifying the δ56Fe value of the SPM near the redox boundary, and also suggests that the iron isotope and the Fe/Al ratio can serve as indicators of hydro-biogeochmical conditions.