We report electrical resistivity, thermopower and Ca K-, Mn L2,3- and O K-edges x-ray absorption near edge structure (XANES) studies on CaMn1−xSixO3−δ with x=0, 0.02, and 0.05. The transport properties measurements indicate that the sample of x=0.02 has the lowest electrical resistivity compared to the samples of x=0 and 0.05. Meanwhile, the temperature dependence of thermopower behaves significantly different for samples of x=0.02 and 0.05. These unusual phenomena are elucidated based on the charge transfer equilibrium principle together with the XANES results, which show that the Si substitution for Mn appears to generate an electron doping effect and lattice distortion. Among the samples, CaMn0.98Si0.02O3−δ exhibits the highest figure of merit of 0.0128 at 300 K. This value represents an improvement of about 52% compared to pristine CaMnO3−δ.