Lentinan, a P-(I - 3)-D-glucan, was isolated from Lentinus edodes by using an improved extraction and purification method to show good water solubility and high yield. The results from 13 C NMR, size-exclusion chromatography combined with multiangle laser light scattering (SEC-MALLS), dynamic light scattering (DLS), and optical rotation revealed that lentinan existed in a triple-helical conformation in the aqueous solution at 25 'C, whereas the thermally induced conformation transition from triple helix to single flexible chains occurred at elevated temperatures. The dependences of the weight-average molecular weight (M,), radius of gyration ((S2),112), hydrodynamic radius (Rh), intrinsic viscosity ([ 7]), and specific optical rotation of lentinan on temperature in 0.9% NaCI aqueous solution showed an abrupt drop at 130-145 'C. It was confirmed that the conformation transitions frorn triple strand to single chain and frorn extended chains to winding chains for lentinan were completed rapidly at 130-145 'C, as a result of the simultaneous destruction of the intra- and intermolecular hydrogen bonds in lentinan. The thermally induced confortnational transition was irreversible. The results from aton- c force microscopy (AFM) and DLS demonstrated the existence of intrachain entanglement for the triple-helical chains, leading to the wormlike linear, circular, and crossover species for lentinan having high M, (1.71 x 101) in aqueous solution at 25 'C.