Thanks for both of your advice.
Actually I also suspected it was the reservoir cap, but then I found that it was not likely. The first day I found the problem again I noticed a strange sound. I heard some 'seeeee....' sound coming out (again I could not locate it exactly, sorry) I suspected it was the cap and the overflow hose, but the overflow tube is not dripping.
For the overflow hose, should it be connected for both ends? I found one end of the hose is connecting to the neck of the reservoir just below the cap, but the other end is not connecting anywhere. Do you know where should it connects to?
The coolant seems to drip from somewhere near the engine. The first time I found the problem I found there were 2 places dripping! One seems to be near the water reservoir, and one seems to be at the front of the engine. But the second time it seems only dripping from the front of the engine.