Can anybody suggest me the easily available part of 51 family micro-controller with A-to-D converter feature in-built.
1) Search in the Keil database for a suitable device, in my observations this is currently the best maintained list.
2) Unless you have very good control and experience in PCB layout, do not use a built in ADC over 8 bits. The logic noise in the analog section will kill you. Without an extremely well laid out PCB and very careful separation of analog and digital ground, decoupling to the right of the two grounds etc. you will not get a good reading beyound 8 bits from a built-in ADC.

Erik

PS
This will be flamed by a few people that have been 'lucky' with above 8 bits, I advise against designing based on luck.