when I disconnect the power supply at transmitter board, my LCD at receiver part shows a number.What's actually happens ?
What you receive is some kind of noise generated by the transmitter... kinda garbage stuff... normal thing.

Do I need to use encoder/decoder IC?
Not suppose to. Can you post your RF module model?

and one more ,how do I test it without RF modules? some tips please?
remove your RF module, and connect your transmitter PORTA.0 directly to your receiver PORTA.0 pin.

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Since you're using True mode, i will suggest you to add the following line in your transmitter code, just before the Main label
Code:
HIGH TransmitterPIN
PAUSE 100
This will ensure your TransmitterPin idle at the right logic level before sending your data.

HTH