I have a differential pressure transmitter which have been fixed at bottom of a liquid tank to measure level inside the tank. One port of the transmitter is open to air.

Can i use normal pressure transmitter in place of it if i don't have diff. pressure transmitter.

 

All pressure transmitters are differential measurement devices, it just depends on the what the reference side is connected to.

What you call a 'normal' pressure transmitter is known as a 'gauge' pressure transmitter where the reference side is always open to atmosphere. That duplicates the functionality of a DP transmitter with its low (reference) side open to atmosphere. (presumably the tank is vented to atmosphere).

What you might run into is getting a gauge pressure transmitter with a range low enough for your application. But if the range is suitable, it's a valid substitution.