La0.833K0.167MnO3 (LKMO) and La0.833K0.167MnO3/SrTiO3 (LKMO/STO) are fabricated at different temperatures. STLK08, STLK10 and STLK12 stand for the samples sintered at 800, 1000 and 1200℃ respectively. X-ray diffraction (XRD) shows that all of the three samples consisted of homogeneous solid solution phases. The resistivity for LKMO shows insulator-metal transition with decreasing temperature, but the resistivities of STLK10 and STLK12 show an insulator behavior over the whole temperature regime (4—300 K). The low-field (0.02 T) magnetoresistance increases from 0.2% to 11% with temperature decreases from 250 K to 4 K for STLK12 sample. The high field (μ0H=5.5 T) magnetoresistance almost increases linearly with decreasing temperature, which is much higher than the magnetoresistance of LKMO (The high field magnetoresistance are 65% and 40% at 4 K for STLK12 and LKMO, respectively). The enhancement in magnetoresistance is quantitatively explained by spin-polarized-tunneling at grain boundaries.