Hi,
i have a problem with my robot. My robot have 2 ping ultrasonic sensor, 2 servo motor and i'm using 6v(1.3mA) battery.
Current draw is measured in A (or mA)
Battery capacity is measured in mAh (milliampere-hour)
Whether it's 1.3 mA or 1.3 mAh, it only tells us that you need a magnifying glass

An AA NiMH Cell has a capacity of roughly 1.5Ah to 3Ah and they can supply a current of around 3A to 10A.
Even small button cells are good for more than 1.3mA (and mAh), so I assume the "m" is just a typo?
The problem is the robot not function properly because of lack of power supply. Should i add 1 more 6v battery and whether i should parallel or series the batteries.
What kind of battery are you using?
Batteries mounted in parallel can be a problem, as one can discharge the other, but if you mount them in series, you'll get 12V!
Much better to get a battery that can handle it on its own - But before you run out to buy a new one, try placing a cap of say 100µF+ directly over your servos and sensors supply leads (as close as possible to the terminals.
All that, assuming you mistyped the battery capacity.
Thx
Dolby