clock of ADC12 in msp430f235

I'm having a bit of trouble to understand what you are exactly asking for - so my answers may not be what you were looking for, if not please rephrase your question:

akhilesh sati said:

How to decide speed of clock for ADC12 in case of msp430f235?

The datasheet states that the ADC12 clock has to be in the range of 0.45 MHz to 6.3 MHz to reach it's specified linearity parameters. The other most deciding factor in choosing the clock speed is mostly a) the wanted sampling and conversion speed and b) the power drawn by the ADC while active.

akhilesh sati said:

Has ADC12 having its different clock for operation?

You can select different clock sources for the ADC12: the ADC12OSC (3.7 MHz to 6.3 MHz typical 5 MHz), the ACLK, MCLK and SMCLK (with the frequency set by you somewhere else)

akhilesh sati said:

In TI's code library in all the program related to ADC12 they did not mention how clock of ADC is confugered by software ?

If none of the ADC12SSEL bits is set in ADC12CTL1 the ADC12 will use the ADC12OSC by default. (That's the case if no special configuration is made) 

I'm not sure to which code library you are referring - MSP430WARE or the code samples available from the product page?