Fractions of a water insoluble α-(1→3)-D-glucan (GL) extracted from Ganoderma lucidum were carboxymethylated (CM) to obtain water-soluble carboxymethylated derivatives (CM-GL) having a degree of substitution (DS) of 0.38～0.51. Weight-average molecular weight Mw and intrinsic viscosity [η] of the samples CM-GL were measured by gel permeation chromatography combined with laser light scattering (GPC-LLS) and viscometry. The CM-GL exhibits a stiffer chain in aqueous solution at 25℃ than the original glucan. The antitumor activities against Ehrlich ascites carcinoma (EAC,5 × 106) of the carboxymethylated derivatives from the α-glucan and curdlan, a β-glucan, are significantly higher than those of the original glucans. The effects of the relatively low molecular weight, expanded chains and better water-solubility of the CM-GL on the enhancement of antitumor activity could not be neglected. The chain stiffness decreased speedily with increase of temperature from 40 to 60℃ or NaOH concentration from 0.1 to 0.4 in the solution, respectively, and the change of the chain stiffness is reversible.