【Abstract】 Magnetoresistance effect was experimentally studied for (Lal-x Yx )2/3Ca1/3MnO3 (x = 0. 195) system with narrow bandwidth. It is shown that the maximum magnetoresistance, which appears near the semiconductor-metal transition temperature (Tc ), increases substantially with magnetic field, e. g., ~ 1. 8×105 % for the field of 7T. We also notice that the magnetoresistance becomes observable even at T　Tc. The origin of the effect is discussed with emphasis upon the spin-polaron theory proposed recently. Main experimental features can be Predicted by this theory, but deviation from experiments increases with magnetic field.更多还原