High-quality (001) thin filmLa0.5Sr0.5CoO3−x(LSCO) has been prepared on (001)SrTiO3substrates by pulsed laser deposition under different oxygen pressures with and without postannealing.C-axis expansion of the LSCO with reducing oxygen pressure was revealed. The electrical resistivity increased over five orders of magnitude when oxygen pressure varied from 1.0 mbar (plus postannealing) to10−3mbar. The negative magnetoresistance (n-MR) property as a function of temperature and oxygen pressure was investigated. Linear dependence of then-MR ratio on magnetic field and temperature was found and significant effect of oxygen stoichiometry on then-MR was demonstrated. The film prepared at 650 °C and 0.1 mbar oxygen shows an-MR ratio of −16&percent; at 81 K under a field of only 0.2 T. ©1998 American Institute of Physics.