We report an investigation into the magnetoresistance (MR) of La0.8Ba0.2MnO3 ultrathin films with various thick-nesses.While the 13 nm-thick film shows the commonly reported negative magnetoresistive effect,the 6 nm-and 4 nm-thick films display unconventional positive magnetoresistive (PMR) behavior under certain conditions.As well as the dependence on the film\'s thickness,it has been found that the electrical resistivity and the PMR effect of the thinner films are very dependent on the test current.For example,the magnetoresistive ratio of the 4 nm-thick film changes from +46％to-37％ when the current is increased from 10 nA to 100 nA under 15 kOe at 40 K.In addition,the two thinner films present opposite changes in electrical resistivity with respect to the test current,i.e.,the electroresistive (ER) effect,at low temperatures.We discuss the complex magnetoresistive and ER behaviors by taking account of the weak contacts at grain boundaries between ferromagnetic metallic (FMM) grains.The PMR effect can be attributed to the breaking of the weak contacts due to the giant magnetostriction of the FMM grains under a magnetic field.Considering the competing effects of the conductive filament and local Joule self-heating at grain boundaries on the transport properties,the dissimilar ER effects in the two thinner films are also understandable.These experimental findings provide an additional approach for tuning the magnetoresistive effect in manganite films.