i want to plug 9v battery to Arduino UNO 2.1mm jack and then, powering l298n drive with Vin pin. is this ok ? can i drive a dc motor like this? or this could cause some trouble? thank you
no you cannot
That should be OK as long as it's not too much current. A motor powered by a 9V battery should be fine.
The current goes through a diode (you can look at the Arduino schematic) which I think is rated at 1 Amp, and the barrel jack & traces on the PCB probably can't handle much more than 1A either.
The diode also has an approximate 0.7V drop across it.
The motor might move for some minutes...
Generally, never power things mike motors, solenoids etc from any Arduino pin.
That's my point, thanks. It will work until the battery dies. Yes, there is a circuit trace connection from the center pin of the battery connection to Vin, but it's a bad direction to go.
I just deleted my previous post in error , it was along the lines of ..
It would be good practice to run a separate supply from the battery rather than risk passing heavy current and possibly voltage spikes through your Arduino .
At some point in the future you may need to do this and this a good time to start good practice
With a diode in it.
It is reasonably assumed that you will not connect a negative voltage to "Vin", but might plug in a backward wired power adapter.
Certainly.
Slip of the wrist. 
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Not really relevant for the user's purpose, but nice to know anyway. Thanks!
İ am very familiar with this circuit. And i know how to do it correctly. But i have a student that age is 9 and he Can not connect cables together. So i want to plugged 9v socket directly to Arduino. Normally i neet to cut socket and add some cables to it which is very hard for my student.
That’s fine as long as you know the consequences and take the risk of destroying the Arduino . The Age of the user doesn’t make it ok .
You could make up a harness with plugs /sockets to ensure things are wired correctly - ( more importantly not wired incorrectly ) you are very limited as to what you can power directly from the Arduino , so this issue is likely to occur again .
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