Today I clawled under the car to have a regular inspection, the engine oil pan was rusted so bad. I used a screw driver to scratch the surface to remove the rust, suddenly it started to leak in one spot. I have no choice but to change the oil pan.
Actually it is not very hard, I only removed engine drive belt plastic cover and the oil pan was easily taken out.
like deadend9009 said, the hardest part is to lose the pan from the engine.