Bicycle Mechanics - A chain problem

I have a road bike which works perfectly before a flat tire happens on the back wheel yesterday. I took off the wheel to change the inner tube last night. Since the back wheel is connected to the chain, so I had to take off the chain as well. But I put everything back on and took it for a test ride, the chain always came off the gear after a couple of cyclings. I realized the chain may have become loose, so I re-install the wheel a few times to make the chain as tight as I can. But still the problem remains.

Anybody has any idea how to fix it? I really appreciate any suggestions and comments.

Is the bike multi speed or single? Does the chain come off in the front or the back?

When you say you took the chain off, did you take the chain apart?

Is the bike multi speed or single? Does the chain come off in the front or the back?

When you say you took the chain off, did you take the chain apart?

It's a single speed, the chain comes off from the back. I didn't take the chain apart.

OK, well in a single speed, the chain should not be as tight as you can make it.

Single speed bikes are quite peculiar in their workings, but the only thing I could suggest would be to make sure your chainline is good, but that is a moot point since it worked before, which would imply the chianoline was good enough.

Try loosing it up a little and see if it helps. Also, if it makes any weird noises that it didn't do before, post that too.

Good luck.

Is the wheel straight in the dropouts? Single speeds are very sensitive to misalignment.