Thank you very much for your reply.That helped a lot. One more question? Do we need a separate power supply? Actually i didnt want to make my assembly heavy with extra power source.I need it to fly of the ground.Is there any way to use common power supply.
You haven't provided details of the DC motor, ie running current under load, and stall current, or the number of motors.
Also the batteries. Their voltage may pull down to an unacceptable level, especially when the motors start or accelerate quickly. What other loads do you have connected besides motor(s) and the Arduino itself?
Generally, you cannot power DC motors directly from an Arduino. It's asking for problems, even with small motors, because more than likely, the startup current surge will exceed the ratings of the Arduino. It can cause unexpected behaviour, and can even reset the Arduino.
You might get away with powering the motor(s) directly from the battery via transistor/MOSFET drivers, with the battery also feeding the Vin of the Arduino, but separate power is the best.
And note that the 1000uS to 2000uS valuesthat I gave are only approximate. You'll need to measure the actual range of pulsewidths from your receiver and set the values in 'map()' appropriately.