I used 6 rechargable AA batteries (total 7.2V) to power my Arduino (UNO R3) Car's two DC Motors (using L293D IC as motor driver). I used separate 9V battery to power arduino itself. But I want to share same 6 battery pack to power my arduino via barrel jack. Please suggest me how can I do this? What precautions Should I take? Like decoupling capacitors or any other?
You may have read that the UNO can accept 7-12 volts as input power. Your AA battery pack (7.2 volts) is very near the low limit of 7 volts. After just a little discharge of the battery (and also voltage drop from motors running), your batteries are likely to deliver less than 7 volts.
As your AA battery goes below 7 volts, then the 5 volts (regulated) will also begin to go lower. A reasonable scenario would be if AA battery was 6.5 volts (1/2 volt low), then the 5 volts regulated would be about 4.5 volts. The UNO is likely to operate ok at this voltage, but as voltage goes lower operation could become a problem depending upon your program used.
Since you are experimenting with a toy car, I would say to go ahead and run both the motors and UNO from the same battery pack. Not a lot to lose on trying.
The UNO already has decoupling capacitors in it's 5 volt regulator circuit.
Just for fun, you could use an Analog input to measure your 5 volts and if the voltage drops to 4.5 volts, you could dis-allow motor operation and flash an LED (or other action). Just a low battery indicator.
If you decide to measure the 5 volts with an analog input, read up on using resistor dividers and the Analog reference (A-ref). Because 5 volts dropping to 4.5 volts could effect your analog reading, depending upon what you use for Analog reference.