A series of LCO epitaxial thin films chemically oxidized by NaClO at 40 degreesC with different oxidation times ranging from 3 to 48 It have been prepared. The results demonstrate that, unlike powder samples, chemical oxidation of LCO thin films shows strong time effect. That is, resistances of the treated films exhibit initially semiconducting behavior and convert into superconducting behavior gradually with increasing oxidation time. With further increasing oxidation time, the resistances return to semiconducting behavior again. This effect was found to originate from the lanthanum deficiency in the topmost layer of the film that causes an insulating layer in the surface of the film, which seriously affects the superconductivity. The origin of lanthanum deficiency is also discussed.